Us news shows that they accept around 400 total. Since they accept about 10% of interviewees outright and they said they would interview around 800-900 this year, I imagine that 80-90 are accepted outright, maybe 100-150 are rejected (15%) and the rest wait listed so maybe around 30-40% are accepted off the waitllist? Don't forget that many people withdraw from the wait list. Also, I spoke to a few students at my interview and they told me that Pitt does this so that they don't give out too many acceptances that are then turned down. They all told me that if I am put on the wait list to write letters of interest. They said not to be too annoying, but to get the point across that I am very interested, if in fact I am.