The game gives you the option to call between 8 and 12 players for each game.
For this reason you can't call all of them.
Remember that every player includes in the call is going to lost energy. In the same way every player that's not included is going to recover it.
For this reason , the teams normally includes 10 players in their calls, for let 5 players recover energy this game.
The energy becomes very important during the season, because is going to affect the way that they are going to play.
Sorry for my English, I tried my best.