It also changes greatly from year to year. Many schools have had years where they have taken >20 off the waitlist and years where they have taken none off the waitlist.
Last year waitlists moved very little as a whole. Hopefully for your sake this year is different. As others have said, nothing will be better than an guess.
At least at all the schools my colleagues and I were accepted or waitlisted at. The waitlisted thread was also very inactive compared to previous years. Which I realize SDN isn't a representative sample but it certainly seemed to be a slow year.