If in doubt, cook too much. Frozen leftovers are a good thing! Last weekend, I cooked for 50 people. I did 5 large bone-in butts (6-8 lbs. each) and 6 slabs of spareribs. The ribs disappeared quickly. Approximately 2 butts' worth of pulled pork was left over. I figure the pulled pork would have fed everyone, but I would have cooked one extra if I hadn't done the ribs, just to be sure.
By the way, I cooked the butts a day in advance. I pulled the pork and vacuum packed it. At the party, I heated it in the oven in foil pans, spritzed with apple juice and covered tightly with foil. It was every bit as good as freshly cooked, and without the stress of wondering if it'll get done in time. Good thing, too, as it took 16 hours to get to 190? internal! I started them at 9:30 PM and took them off at 1:30 PM the next day (the day before the party).
Hope this helps.
Steve