You can play in any formation, providing you have a minimum of 1 wicket-keeper, at least 3 batters, at least 2 all-rounders and at least 3 bowlers selected in your team at all times.
Every fantasy game can have a different game structure.
Default game structure allows 11 players per team, however, games can have a 6, 7, 8, 9 or 10 player structure.
Notice on the pitch to see 0/11, or 0/6 (or same) to know the game structure of the game.