I guess it depends on how enthusiastic they will be on finding the characters. I know that's not much help. The Mouse for Less site (sorry I can't include the address - I don't have enough posts yet ) has a list of characters - there are more than I imagined! So I suppose you could count them up and use that for the number of pages?
Our first trip is also next month and what I am doing is using an 8x8 scrapbook (that we can add pages to) and just taking the 8x8 cardstock for the characters to sign. I figure they'll be able to get more than 1 per page or even use front and back (since they will be in page protectors and both sides will be visible). I'll probably take about 75-100 sheets. I got some really cute albums at Walmart, in the scrapbooking section. They are black with a black outline of Mickey embossed on the front.