The BBQ restaurant that our church often uses says to plan on getting 5 sandwiches per pound of pulled pork.
However, I know that our church kitchen supervisor likes to be on the safe side so he'll add 50% to how ever many folks he thinks is coming, assuming one sandwich per person. In other words, for 100 folks, he would order 30 pounds of pulled pork. That's with no white bread or corn bread served, just buns, as most folks around here seem to prefer the pulled pork on a sandwich.
Regardless, I can't remember what the average weight loss is during cooking, but I'm almost positive that for just twenty pounds of 'Q, you'll still need to cook more than thirty pounds of pork butt. Hopefully you'll see some caterers chime in here.
Dave
UDS, wsm, wots, char-griller