What happens to the seats that ppl who dont accept their acceptance letter form a UC?

All colleges accept way more people than they can enroll, because they know that a lot of people aren't going to accept their admission offers. So they estimate what percentage of people will actually go, and decide how many offers to give out according to that. Most the UC's also have waitlists now, so if they are able to accept some extra people, it's going to be out of the people who originally were placed on the waitlist.