On our trip last year we bought the regular ones from the parks that have about 100 blank pages in them. After seeing most of the characters in the parks, plus a few character meals, we almost filled the whole thing! I think there was only maybe 10 blank pages left.
Granted, there were a couple doubles by accident, but we also have a few like Chip and Dale that signed side to side.
So...depending how 'character crazy' you think your family will be.......Also, I don't know how long you are staying, but we normally go for about 12 days at a time so we plan in lots of character time.
Worst case scenario, you make a book for the most important characters, leave a few blank pages, and if it fills up while you still have ones you'd like to see buy one with about 20 or so pages at the World.
Hopes this helps!