there is probably a set limit to how many players you can have but really it probably has to do with what the general manager and coaches want and what fits teir salary cap.
most teams in the mlb carry around 40 to 60 only playing 9 starters. In little league there is usually 12 to 13 on a team ..Each MLB team has a 40 man roster and 25 of them are active at the MLB...
9 players are on the field at a time.
The positions are;
-right field
-center field
-left field
-first base
-second base
-third base
-shortstop
-pitcher
-catcher