I can't say anything about how many they accept on December 1 (they said it switches based on the year), but I can talk about the interview. The other interviewees I spoke to during my interview day essentially claimed to have similar experiences to the ones I did, but I can't say if that is a rule. There are two interviewers (faculty and student) who read your file beforehand. They ask a few preplanned questions (you may want to ask around about those) to every applicant and ask if you have any questions. The consensus I got from other applicants was that their interviewers didn't seem so enthusiastic at the end of the interview; it was a bit stilted. I have no idea whether that is a bad thing or not, as I don't know who will get in. They were nice enough, but I didn't feel extremely confident leaving the room because of that point.
