Your squad is the foundation of your club. You can have a maximum of 30 players, starting off with a maximum of 22. 

Squad overview

The other eight slots in total can be gained from Tell-a-friend feature (max 2 per club) and for example by earning specific achievements or they can be bought during special promotions (max 4 per club). 
Two extra squad slots can be obtained through stadium upgrades after you purchase "Squad slot Plus (common)" and "Squad slot Plus (rare)" stadium upgrades.

The better your squad, the better the chance to win titles, like your league championship.

Always make sure that your team is well-balanced, to receive full efficiency in your training. On the positions defence, midfield,on the wings and in the offence, you should`t have no more than seven players, or the training efficiency goes down. For the goalkeepers it is limited to three players without losing efficiency.