So I still haven't found an exact answer on how many schollies USC has, but I think they are limited to 75 total and 15 per year. They can take 18 in this class because there are 3 guys that can counted towards last year. So it shouldn't matter how many players leave the team, they are still limited in recruits.
You answered your own question. They only have room for 18 3 more then they should because they had 3 early enrollees that didnt count towards the 2012 class. Unless someone transfers, gets kick off the team, loses a scholarship, or mysteriously goes missing, they only have room for 18 right now. Of course they will find a way to get a guy like Priest if he wants in, either kick a guy off their current class (one WR already tore his ACL) or make a guy transfer. Its BS but they will find a way to take Nkemdiche and Priest both if they both want in.
The NCAA is going to have to make scholarships a 4 year contract, that will help teams from doing things like that. Of course they will just find a way around that as well.