Squad size limits

From RS Wiki

You must have at least 14 players in your squad (at least 1 goalkeeper). These 14 must not include loaned players and youths. You're not able to fire, sell or let go any player if your squad size will become less than 14 or you be out of goalkeepers as a result.

In the other hand, your squad must not have more than 30 players. The 30 players limit includes all non-youth players you own plus the ones you've loaned in or out (as long as they're not youths), plus the pending buying or youth proposals you have. For example, suppose you have 25 non-youth players in your list. From these, 2 are loaned in from another team. Besides that, you have 4 players loaned out (non-youths) and a contract proposal to one of your youths. The question is, can you buy a player in this situation? The answer is no, as you've reached the 30 players limit (25 + 4 loaned out + 1 open proposal).