There is a limit to how many players you can have in your team, but you can still store up to a maximum of 500 players in your Clubhouse.
Any player kept in the Clubhouse can't be trained or used in matches.
If you want to train a player or use a player in a match, first you need to send that player to your reserves.
To send players from your Clubhouse to the Reserves, just select the player(s) in question and tap 'Send Players to Reserves'.
* This won't work if you don't have room in your Reserves left.