Last year the CEU waiting list remained unchanged; it only included those who had been accepted in the two admission rounds. The fact that there are now three rounds is new this year.
In Madrid, the university entrance exam (EvAU) was very difficult and demanding, and many Madrid residents, whose backup plan was CEU and who had it as a reserve, didn't get in. But that's the only time it's happened, and it shouldn't happen again.
If this year is normal, if you're 53rd on the waiting list you'll probably get in.