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.
beidh ar la linn.
wisdom is often in short supply within ones' ego.