fishon, it depends on what cut of pig you have, and what else is being served, but 30 lbs should be plenty.
for my dept's christmas party this past year, i made a half sevice tray of bbq style, and a half service tray of carolina style pulled pork that fed around 75 to 80 people. both deep 1/2 trays were packed to the top and were so heavy that i needed to double the aluminum trays up so they wouldn't bend or collapse.
i made 2 picnic roasts and 2 butts, i'm guessing around 7 or 8 lbs each. i used 1 of each cut in each 1/2 tray of pulled pork, so maybe around 22 to 24 pounds in total.
you get a slightly different flavour from each cut, and i've heard so many different opinions on who prefers what, so i combined 1 of each.
there's a lot more waste from skin and bone on california picnics so they're often much cheaper.
i slow cooked mine with onions, so that helped stretch the meat out a bit as well when the cooked onion was chopped and added to the meat.
one trick i learned: if you're serving them with bread, serve them on mini-buns, aka slider rolls if you think you may not have enough for everyone.
Portion size is so very important. Are they all men? 1/2 men 1/2 women? Kids in there?
I'm think'n smoke'n so my #'s may be off, but for smoke'n I'dd throw another 2 butts on the smoker. Look'n at 46#'s, about, fresh uncooked. So many variables. I'd rather have plenty and save it than turn people away.
Chances are 30 will do, but heck, the pit crew will eat 10#'s alone!
If cooked...30 lbs will yield 120 4oz servings ~~~ 120 Ok sandwiches..
If cooked...30 lbs will yield 80 6oz servings ~~~ 80 Nice sandwiches.
If Raw.. After trimming, shrinkage during cooking, excess fat, pulling loss, bone etc your yield will be in the 40-60% range...A 8 lb pork butt will yield roughly 3-5 pounds of usable meat.....Figuring a 50% yield ( 4 lbs.of usable meat) ~~ will give you 16 4 oz. sandwiches...or 11 6 oz sandwiches....
The question you need to answer is how many sandwiches will your 100 people eat.