Disney won't release those numbers. You're going to hear a difference in opinions greatly on this too.
I went Halloween night last year-- also a sell out. A few years ago I went during Easter week for another trip-- I felt the crowds were comparable. I personally thought it was a massive sea of people and hard to navigate. Now the interesting part is that most ride lines were relatively short. Under 15 minutes-- some even walk on. My group of people eneded up treating it more like a park day as we couldn't really get near the "special" events. We did go to the 2nd parade (but to sit on Main St we still had to get there 75 minutes early- by 1 hr to parade time-- all the sitting spots and much of the first row standing was filled in. The "rare" character lines were very, very long. We also found the candy lines to be very long-- at one my aunt and I started counting the number of people in the one line by the POC/Jungle Cruise and we stopped at 135-- and the line was still going beyond what we could see.
Now aside from that -- I did really enjoy
MNSSHP. Planning on doing it again in 2005-- for us though it won't be Halloween again. Rather early or mid month. Plan accordingly--and have fun with it. The bright side at 11:15 pm- 11:45 pm we walked onto every open Fantasyland attraction w/ NO wait-- including Dumbo, Pooh, Snow White and Peter Pan!!!!!!
One last thing-- it was really hot and humid-- so dress accordingly too.