I believe soon after the final interview cycle is over, they will then await for final responses from those who are to either accept / put a deposit down or decline acceptance.
From that point they will go over all alternate list candidates (those who interviewed and were put on the alternate list). I believe the number of people is either high 80's or low 90's. The adcom will then meet and go over all the applicants and decide who they will offer a acceptance too. By my understanding, at this point it is not ranked, they will go over all alternate candidates. I suspect we must be down to 10-15 seats by now, but I am sure there will be some people who will withdraw last minute as well.
Hope this helps.