Question: how many pitchers are in a rotation?
About the article "Baseball", section "Player rosters".

Candidate passages:
 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. 
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. 
There are many ways to represent a rotation.
It is useful to understand how different representations relate to one another, and how to convert between them. 
In baseball statistics, Batting average on balls in play (abbreviated BABIP) measures how many of a batter’s balls in play go for hits, or how many balls in play against a pitcher go for hits, excluding home runs.
BABIP is commonly used as a red flag in sabermetric analysis, as a consistently high or low BABIP is hard to maintain—much more so for pitchers than hitters.
Therefore, BABIP can be used to spot fluky seasons by pitchers, as with other statistical measures; those pitchers whose BABIPs are extremely high can often be expected to improve in the following season, and those pitchers whose BABIPs are extremely low can often be expected to decline in the following season.
A normal BABIP is around .300, though the baseline regression varies depending on number of factors including the quality of the team's defense (e.g., a team with an exceptionally bad defense might yield a BABIP as high as .315) and the pitching tendencies of the pitcher (for instance, whether he is a groundball or flyball pitcher).
While a pitcher's BABIP may go up and down in an individual season, there are distinct differences between pitchers' career averages.
The equation for BABIP is:formula_1where H is hits, HR is home runs, AB is at bats, K is strikeouts, and SF is sacrifice flies.