Question: How many pitchers constitute the team's pitching rotation or starting rotation?
About the article "Baseball", section "Player rosters".

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Player rosters.
Roster, or squad, sizes differ between different leagues and different levels of organized play.
Major League Baseball teams maintain 25-player active rosters.
A typical 25-man roster in a league without the DH rule, such as MLB's National League, features:
eight position players—catcher, four infielders, three outfielders—who play on a regular basis
five starting pitchers who constitute the team's pitching rotation or starting rotation
six relief pitchers, including one specialist closer, who constitute the team's bullpen (named for the off-field area where pitchers warm up)
one backup, or substitute, catcher
two backup infielders
two backup outfielders
one specialist pinch hitter, or a second backup catcher, or a seventh reliever
 A starting pitcher in professional baseball usually rests three or four days after pitching a game before pitching another.
Therefore, most professional baseball teams have four or five starting pitchers on their roster.
These pitchers, and the sequence in which they pitch, are known as "the rotation."
In modern baseball, a five-man rotation is most common.
Often a manager identifies pitchers by their order in the rotation, "number 1," "number 2," etc.
"Discussions over whether Jason Schmidt or Brad Penny is more deserving to occupy the No.
2 spot in the starting rotation behind Derek Lowe can cease, as least temporarily."
In baseball or softball, a starting pitcher or starter is the first pitcher in the game for each team.
A pitcher is credited with a game started if they throw the first pitch to the opponent's first batter of a game.
A pitcher who enters the game after the first pitch of the game is a relief pitcher.
Starting pitchers are expected to pitch for a significant portion of the game, although whether they do can depend on many factors, including effectiveness, stamina, health, and strategic reasons.
A starting pitcher in professional baseball usually rests three or four days after pitching a game before pitching another.
Therefore, most professional baseball teams have four or five starting pitchers on their rosters.
These pitchers, and the sequence in which they pitch, is known as the "rotation".
In modern baseball, a five-man rotation is most common. 
Pitching is physically demanding, especially if the pitcher is throwing with maximum effort.
A full game usually involves 120–170 pitches thrown by each team, and most pitchers begin to tire before they reach this point.
As a result, the pitcher who starts a game often will not be the one who finishes it, and he may not be recovered enough to pitch again for a few days.
The act of throwing a baseball at high speed is very unnatural to the body and somewhat damaging to human muscles; thus pitchers are very susceptible to injuries, soreness, and general pain.
Teams have devised two strategies to address this problem: rotation and specialization.
To accommodate playing nearly every day, a team will include a group of pitchers who start games and rotate between them, allowing each pitcher to rest for a few days between starts.
A team's roster of starting pitchers are usually not even in terms of skill.
Exceptional pitchers are highly sought after and in the professional ranks draw large salaries, thus teams can seldom stock each slot in the rotation with top-quality pitchers.
The best starter in the team's rotation is called the ace.
He is usually followed in the rotation by 3 or 4 other starters before he would be due to pitch again.
Barring injury or exceptional circumstances, the ace is usually the pitcher that starts on Opening Day.
Aces are also preferred to start crucial games late in the season and in the playoffs; sometimes they are asked to pitch on shorter rest if the team feels he would be more effective than the 4th or 5th starter.
Typically, the further down in the rotation a starting pitcher is, the weaker he is compared with the others on the staff.
The "5th starter" is seen as the cut-off between the starting staff and the "bullpen".
A team may have a designated 5th starter, sometimes known as a spot starter or that role may shift cycle to cycle between members of the bullpen or Triple-A starters.
Differences in rotation setup could also have tactical considerations as well, such as alternating right- or left-handed pitchers, in order to throw off the other team's hitting game-to-game in a series.
Teams have additional pitchers reserved to replace that game's starting pitcher if he tires or proves ineffective.
These players are called "relief pitchers", "relievers", or collectively the "bullpen".
Once a starter begins to tire or is starting to give up hits and runs a call is made to the bullpen to have a reliever start to warm up.
This involves the reliever starting to throw practice balls to a coach in the bullpen so as to be ready to come in and pitch whenever the manager wishes to pull the current pitcher.
Having a reliever warm up does not always mean he will be used; the current pitcher may regain his composure and retire the side, or the manager may choose to go with another reliever if strategy dictates.
Commonly, pitching changes will occur as a result of a pinch hitter being used in the late innings of a game, especially if the pitcher is in the batting lineup due to not having the designated hitter.
A reliever would then come out of the bullpen to pitch the next inning.
When making a pitching change a manager will come out to the mound.
He will then call in a pitcher by the tap of the arm which the next pitcher throws with.
The manager or pitching coach may also come out to discuss strategy with the pitcher, but on his second trip to the mound with the same pitcher in the same inning, the pitcher has to come out.
The relief pitchers often have even more specialized roles, and the particular reliever used depends on the situation.
Many teams designate one pitcher as the "closer", a relief pitcher specifically reserved to pitch the final inning or innings of a game when his team has a narrow lead, in order to preserve the victory.
Other relief roles include set-up men, middle relievers, left-handed specialists, and long relievers.
Generally, relievers pitch fewer innings and throw fewer pitches than starters, but they can usually pitch more frequently without the need for several days of rest between appearances.
Relief pitchers are typically guys with "special stuff".
Meaning that they have really effective pitches or a very different style of delivery.
This makes the batter see a very different way of pitching in attempt to get them out.
One example is a sidearm or submarine pitcher. 
Defensive strategy.
Effective pitching is vitally important to a baseball team, as pitching is the key for the defensive team to retire batters and to preventing runners from getting on base.
A full game usually involves over one hundred pitches thrown by each team.
However, most pitchers begin to tire before they reach this point.
In previous eras, pitchers would often throw up to four complete games (all nine innings) in a week.
With new advances in medical research and thus a better understanding of how the human body functions and tires out, starting pitchers tend more often to throw fractions of a game (typically six or seven innings, depending on their performance) about every five days (though a few complete games do still occur each year).
Multiple pitchers are often needed in a single game, including the starting pitcher and relief pitcher(s).
Pitchers are substituted for one another like any other player (see above), and the rules do not limit the number of pitchers that can be used in a game; the only limiting factor is the size of the squad, naturally.
In general, starting pitchers are not used in relief situations except sometimes during the post-season when every game is vital.
If a game runs into many extra innings, a team may well empty its bullpen.
If it then becomes necessary to use a "position player" as a pitcher, major league teams generally have certain players designated as emergency relief pitchers, to avoid the embarrassment of using a less skillful player.
In baseball's early years, squads were smaller, and relief pitchers were relatively uncommon, with the starter normally remaining for the entire game unless he was either thoroughly ineffective or became injured; today, with a much greater emphasis on pitch count, over the course of a single game each team will frequently use from two to five pitchers.
In the 2005 ALCS, all four of the Chicago White Sox victories were complete games by the starters, a highly noteworthy event in the modern game.
While delivering the ball, the pitcher has a great arsenal at his disposal in the variation of location, velocity, movement, and arm location (see types of pitches).
Most pitchers attempt to master two or three types of pitches; some pitchers throw up to 6 types of pitches with varying degrees of control.
Common pitches include a fastball, which is the ball thrown at high speed; a curveball, which is made to curve by rotation imparted by the pitcher; a change-up, which seeks to mimic the delivery of a fastball but arrives at significantly lower velocity; a splitfinger fastball, which attempts to mimic the delivery of a fastball, but has slight topspin rotation allowing the baseball to sink as it approaches the batter due to the position of the index finger and middle finger being "split" on the baseball; a slider, which is made to curve laterally by rotation imparted by the pitcher; a knuckleball, which is held either by the knuckles closest to the nails or by the nails themselves allowing the pitcher to throw a baseball with little or no spin on the ball allowing the baseball to move in any direction; a Vulcan changeup, which seeks to mimic the delivery of a fastball but arrives at a significant lower velocity due to the pitcher holding the baseball with the middle and ring fingers slightly apart; a circle changeup, which is held in the palm of the hand with the index finger curled around the side of the baseball, allowing the pitcher to mimic the delivery of a fastball but arrives at a significantly lower velocity.
To illustrate pitching strategy, consider the "fastball/change-up" combination: The average major-league pitcher can throw a fastball around 90 miles per hour (145 km/h), and a few pitchers have even exceeded .
The change-up is thrown somewhere between .
Since the batter's timing is critical to hitting a pitch, a batter swinging to hit what looks like a fastball, would be terribly fooled (swing and miss, hopefully) when the pitch turns out to be a much slower change-up.
Some pitchers choose to throw using the 'submarine style,' a very efficient sidearm or near-underhand motion.
Pitchers with a submarine delivery are often very difficult to hit because of the angle and movement of the ball once released.
Walter Johnson, who threw one of the fastest fastballs in the history of the game, threw sidearm (though not submarine) rather than a normal overhand.
True underhanded pitching is permitted in Major League Baseball.
However, it is difficult to generate enough velocity and movement with the underhand motion.
Among Major League pitchers today, Chad Bradford has the closest to an underhand delivery, with his knuckles sometimes scraping the ground.
However, he is still usually considered a "submarine" pitcher.