I have 21 players on my roster and the free youth for a total of 22.

Everyone plays everyday since I always play 3 games a day. If the player doesn't appear in the league/cup/champions league game, he plays in the friendly.

My squad:
3 ST
1 AML/AMC
1 AMR/AMC
1 ML/DML
1 MR/DMR
4 MC
2 DC/DMC
3 DR/DC
3 DL/DC
2 GK

That allows me to play every formation and to have a sub for every position (since I count players who have a yellow caution as a good sub)