Depends on the league. In MLB it is 9 on the defense at one time.
For street baseball, 4 players is enough: one person for serving the ball, one with the batt, and the other two accompenying them (trying to catch the ball when the batt-guy hits the ball).
Well there are only 2 players, one for white and one for black.
it is 9 that can play at one time but each team always has more players. In Major League Baseball, a team has 25 players that are able to play in each game.
Going back to 1871, out of 17172 baseball players, 8240 have pitched in at least one game, 48%