Question: What is the number of players that can play racquetball at once?
About the article "Racquetball", section "Game variations".

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Paddleball can be played with two players (singles), three players ("cut throat"), or four players (doubles).
The rules of paddleball are similar to indoor racquetball, and both sports are played on the same court.
The most-significant differences between paddleball and racquetball are:  Paddleball players play with a solid paddle, rather than a strung racket.
A paddleball is slower (and slightly larger) than a racquetball.
Paddleball games are played to 21 points, instead of 15 or 11 (as in racquetball).
There are other minor differences, but racquetball players tend to pick up the sport quickly and many players are good in both.
Marty Hogan, Charlie Brumfield and Bud Muehleisen, for instance, each held national open titles in both sports; Hogan held both national open titles during the same year.
The differences in the paddle and the ball make for longer rallies than in racquetball, and use more of the court.
As a consequence paddleball tends to be more physical, and contact between players (while discouraged) occurs more often than in racquetball.
Playing the sport at the highest level requires an advanced degree of fitness and endurance, similar to that required by squash.
The official governing organization for paddleball is the National Paddleball Association (NPA), whose website is the official source for current rules and tournament schedules. 
The International Racquetball Federation (IRF) governs the World Racquetball Championships, which were first held in 1981 in conjunction with the first World Games.
The second World Championships were played in 1984, and since then have been held biennially in August.
Players from the United States have won the most World Championship titles.
The IRF also runs the World Junior Racquetball Championships that occur annually in December, as well as the annual World Senior Racquetball Championships for players who are 35 years of age or older.
Racquetball has been included in the World Games on five occasions: 1981, 1989, 1993 and 2009.
Racquetball has been included in the Pan American Games in 1995, 1999, 2003, and 2011.
Toronto will host the 2015 Pan American Games, and they plan to have racquetball on the program.
There are three professional racquetball organizations.
The International Racquetball Tour (IRT) is the men's professional organization.
IRT tournaments occur usually in the USA, with some events occurring in Mexico and Canada.
The Women's Professional Racquetball Organization (WPRO) is the women's professional organization, while the Classic Pro Racquetball Tour (CPRT) is for players 40 and older many of whom were pro players at younger ages. 
 "For other sports often called "paddleball", see Paddleball (sport)."
Racquetball is a racquet sport played with a hollow rubber ball in an indoor or outdoor court.
Joseph Sobek is credited with inventing the modern sport of racquetball in 1950 (the outdoor, one-wall game goes back to at least 1910 in N.Y.C.
), adding a stringed racquet to paddleball in order to increase velocity and control.
Unlike most racquet sports, such as tennis and badminton, there is no net to hit the ball over, and, unlike squash, no tin (out of bounds area at the bottom of front wall) to hit the ball above.
Also, the court's walls, floor, and ceiling are legal playing surfaces, with the exception of court-specific designated hinders being out-of-bounds.
It is very similar to 40×20 handball, which is played in many countries.
Joe Sobek is credited with inventing the sport of racquetball in the New Britain Connecticut YMCA, though not with naming it.
A professional tennis and handball player, Sobek sought a fast-paced sport that was easy to learn and play.
He designed the first strung paddle, devised a set of rules, based on those of squash, handball, and paddleball, and named his game "paddle rackets".
In February 1952 Sobek founded the National Paddle Rackets Association (NPRA), codified the rules, and had them printed as a booklet.
The new sport was rapidly adopted and became popular through Sobek's continual promotion of it; he was aided by the existence of some 40,000 handball courts in the country's YMCAs and JCCs, wherein racquetball could be played.
In 1969, aided by Robert W. Kendler, the president-founder of the U.S. Handball Association (USHA), the International Racquetball Association (IRA) was founded using the name coined by Bob McInerney, a professional tennis player.
That same year, the IRA assumed the national championship from the National Paddle Rackets Association (NPRA).
In 1973, after a dispute with the IRA board of directors, Kendler formed two other racquetball organizations, yet the IRA remains the sport's dominant organization, recognized by the United States Olympic Committee as the American national racquetball governing body.
In 1974, the IRA organized the first professional tournament, and is a founding member of the International Racquetball Federation (IRF).
Eventually, the IRA became the American Amateur Racquetball Association (AARA); in late 1995, it renamed itself as the United States Racquetball Association (USRA).
In 2003, the USRA again renamed itself to USA Racquetball (USAR), to mirror other Olympic sports associations.
Kendler used his publication "ACE" to promote both handball and racquetball.
Starting in the 1970s, and aided by the fitness boom of that decade, the sport's popularity increased to an estimated 3.1 million players by 1974.
Consequent to increased demand, racquetball clubs and courts were founded and built, and sporting goods manufacturers began producing racquetball-specific equipment.
This growth continued until the early 1980s, and declining in the decade's latter part when racquet clubs converted to physical fitness clubs, in service to a wider clientele, adding aerobics exercise classes and physical fitness and bodybuilding machines.
Since then, the number of racquetball players has remained steady, an estimated 5.6 million players. 
The Italy national racquetball team represents the Federazione Italiana Racquetball in racquetball international competitions.
Is a member of the European Racquetball Federation and International Racquetball Federation. 
The purpose of the game is to hit the ball (either a racquetball, or a spaldeen or kick pinkie) in such a way that the opponent cannot return the shot before its second bounce after hitting the wall.
In its simplest form two players can play for points.
More players can play an elimination game or Ace-King-Queen.