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.
in nomine patri, et fili, et spiritus sancti.
Meh nom eh noh...doot dooooo do do do.