See below photo for roughly the amount of pulled pork to be in each sandwich.
My guess is 18-20 sandwiches per 8lb (raw weight) butt.
I always figure 6 ounces of sandwich meat per sandwich. So 100 sandwiches would be 600 ounces of meat or 37.5 pounds. So with roughly 50% shrinkage you need to start with about 75 to 80 lbs of raw pork butt. Sounds like about 8 to 10 average pork butts.