How many baseball players on field at one time?

The maximum number of players on the field at any one time is 13, nine defensive players including the pitcher, and four offensive players, one occupying each base and the hitter. The catcher, by the way, is the only defensive player allowed to be in position not in fair territory.

If your questions refers to how many players are allowed in the "outfield" at any one time, the answer is there is no limit, all eight defensive players, including the catcher, if you so choose, may be positioned in the outfield. The only one not allowed is the pitcher, of course, who must pitch the ball to begin play.