I think ours was about 14
To the best of my recollection I would guess around 20-25. The huge benefit I found to doing the earliest tour was that Brian made sure our group was the first early entrance group to view the book. Each of us was able to have an unobstructed view for a short period of time. Even twenty minutes later it was hard to get an unobstructed view as twice as many people were in the exhibit.