There are 11 players on the field per side, goalies included. The only way this number changes is if a player is sent off by the center referee for a red card during the game. That team would then have to play short. The only exception is if the red card was given before the game, the player would then be allowed to be replaced and the team could play with a full team.
11 players can play on a field at a time. The MLS roster has 20 players.11
At about 490
At about 500