From reading previous posts in this thread, they interview ~60 students per day. In the email I received, I was offered 2/21 and 3/7 as dates for the waitlist. Someone in this thread mentioned that administration was looking for tour guides for three dates. If we assume that they're only interviewing on 2/21, 3/7, plus 1 more day, there could be as many as 180 students interviewing for the waitlist. Someone else mentioned that they wait list about 5-10 students per 60 students interviewed at the "normal" interviews. I found 9 known interview dates plus 1 early decision day while scrolling through this thread, so there could be as many as 90+180=270 students total on this waitlist, assuming that no one withdraws from the waitlist. This is how I considered it.