hmmm, I had over 20, I counted at the time but I forget the exact number. It's a big group, well organized though. It does make for a long wait between interviews though (but you get to hang out in the student lounge, so it's okay)
The day I interviewed, there were 24 ppl, exactly the number of seats they have in the room where they give the presentation. The guy said that on that particular day, everyone they invited showed up but usually he says there is about 21-22.