It may typically be 15, but I'm certain there has already been some movement. Last year I was initially at #17 or 18 on the waitlist, but I found out later that applicants had already withdrawn their name from the list, so I was added late relative to other people. IIRC I moved up to #10 by June. They may put 20-25 on the waitlist in total, but if you're one of the first 10 then you have an excellent chance of being offered an acceptance.
It isn't clear to me: did you get off the waitlist last year? And choose to attend a different school?
Appreciate your information and answers btw it's helping me make my decisions regarding my other matriculation opportunities.