You can have as many players as you want on your list but you can only sell them or use them in games or training if theyre signed, and you can sign only up to 22.
Unsigned players (youths or excess players won in auctions after your squad is full) will leave after 7 days so you got plenty of time to sell players if you want to buy the youth player for tokens.
at least thats the way it works if you buy too many players in auctions... not 100% sure what happens with youth players.