For Major League Baseball, there is a 40-man roster, in which 25 are on the "active roster" and the rest either on a temporary disabled list or in a minor league position.
In a baseball game, only 9 members of the fielding team are in fielding positions, but can be replaced by another player, or can be rotated to a new position during the game. The 9 positions are :
Shortstop (plays between 2nd and 3rd)
So the total possible number of players on the field is between 10 and 13, including the batter for the opposing team and as many as 3 batting players occupying the bases.