Interesting thread.
I usually have..
3 GK's - Mainly because I always like to have a keeper on the bench just incase, if one gets injured and you only have 2 you could lose a game if you're other one gets injured during the game.
4 CB's - Just for versatility really, I change my centre back for different strikers facing them and the way teams play, e.g if a team plays wide I'll play tall strong centre backs who are immense in the air, e.g Kaboul, if a team plays narrower I'll play more ball playing centre backs so they can nice balls out to the wingers to find the oppositions weakness which in this case would be a lack of width. & Obviously if a team has fast strikers = Fast centre backs and vice-versa.
4 FB's - Basically the same as centre backs, need 2 defensive one's and 2 attacking one's, all capable of playing in the first team. Full backs are really important in league's like the Premier League, especially when at home.
6 CM's - Basically because I might play 4-5-1 and I always like to have 2 CM's on the bench incase I need to change my approach to the game if I'm leading or a team makes changes to go more attacking etc.
4 Wingers - Need ball playing, technical wingers, aswell as fast pacey wingers with great dribbling. Once again all about versatility and being able to change it in the game if an approach isn't working.
2 DM's - Don't usually use them, but it's good to have them incase I decide to use them for a match against a stronger team.
2 AM's - Same as DM's really, but I tend to use these more, so if I buy one, I go for top quality one's.
5 ST's - I usually only use 1 or 2 every game, but strikers seem to get injured on my saves more than anyone else, so I need a lot. Plus once again it's about versatility and about having different types of strikers, e.g having a player like Crouch on the bench if you're starting a player like Defoe is good, as you can completely change you're type of striker. Tottenham is a prime example of this, usually why they score against me as it's so hard to adapt to their strikers sometimes.