I recently smoked seven pork butts for a friend's farewell party. There were about seventy people in attendance. We didn't even touch the last three pork butts and we still had plenty left over from the first four. I would suggest going with five at about 8 lbs each as Steve suggests.
Interesting side note on my big cook, one of the guys at the party I cooked for has been a vegetarian for over two years. He fell of the vegetarian wagon for some good smoked pork butt. I'm not sure if it something I should be proud of or feel guilty for, but I took it as a complement nonetheless.
-Scott