Having applied twice to schools and been on maybe 6 wait lists, I think you are correct in asking for only an rough estimate. I was on a wait list at three schools last year (Iowa, Case and Hopkins) and each told me a different story about numbers.
Iowa had two wait lists, one for in state and one for out of state. I was number 92 on the out of sate one. They did tell me though that the year before, they had gone through 78 on their out of state list.
I remember also that Hopkins told me that they rarely got through their first third group on the wait list meaning that the majority of the time, two thirds of students were placed on the waitlist for no real reason.
While all of this might seem kind of dreadful, I should mention that I just did get off the waitlist this year at Penn...
Good luck.