I am guessing 30ish based on emails with the DGS, but they wait list by area (ethics, cogsci, etc.). If some ethics guy they accepted drops out, then someone on the ethics waiting list gets in. Someone doing cogsci who they accepted declines the offer, someone on the cogsci list gets in. So you might be on a shorter area of interest waiting list, 5-7 people or something. But then, there might only be 1 ethics "slot". They have 4-6 slots to fill overall and they thought that almost 40/170 applicants were qualified. They accepted more than 4-6 people, assuming someone would drop out, I guess. I think that means that even if people turn down their offer, there is no guarantee that you'll be bumped up on the wait list. But who knows? Maybe we'll get lucky. I am wait-listed at WUSTL too, and at Indiana.