I'm in with the 8 butt number and here's why... I can give you all kinds of formulas, and can tell you stories about debates on yield of pork butts..... 60/40, 65/35 and so on, but I always guestimate in favor of having a little more meat than I need. My magic number is 50/50. In other words an 8 pound raw butt will yield 4 pounds of pulled meat.
8 ounce portions are very generous, 6 ounces is closer to the mark with some crowds and I've seen plenty of times where 4 ounces works.... but you don't want to run out.
But just for conversation (and demonstration) Using 5 ounces per portion X 100 portions = 500 ounces. Divide that by 16 ounces per pound, and you would need 31-1/4 ounces of cooked meat. If you loose 50% from raw weight..... you need to start with 62-1/2 pounds of butts. Which is about 8 average butts, which come two to a pack.
If you are serving the PP in sandwiches, have someone dish up the meat, this also helps with portion control. Buy small hamburger buns, or loaves of white Texas toast. You can stack meat higher and it looks like a bigger portion. And don't buy huge paper plates. Actually the ones with the compartments work great if you have sides. Make sure all of the sides are attractive looking, and most important, really, really the most important thing.... have the meat at the end of the serving line. If one guy sees another guy make two sandwiches and skip all the sides, he wants two. Heheheee.
Have 2 or 3 kinds of sauce for folks to add themselves. Coleslaw goes great on PP sandwiches (and makes the meat go farther). Any thing pickled also goes great with PP. A huge jar of pickles, some pickled onions or cucumbers are great to have right at the meat serving station.
