Here's what happens:
The school will invite about 6-9 applicants for each seat.
Given last minute cancellations, travel delays, etc, maybe 5-10% of invitees will be no-shows.
The school will make offers to somewhere between 1.1 and 3 applicants for each seat to be filled.
The remaining applicants will be waitlisted or declined. Some schools will decline only those applicants who were really odd or inappropriate at the interview and put almost everyone on the waitlist (big waitlist in relationship to number interviewed). Other schools will keep a smaller waitlist and put the rest of the pool out of its misery.
If a school makes few initial offers, they may take many applicants from the waitlist in order to fill the class. If a school has a "bad year" and most everyone who gets an offer turns it down, then they will take a large number from the waitlist. Some schools will make enough initial offers to fill (or over fill) the class and do not need to go to the waitlist.