assuming 1/4 pound per person, and 8 pound butts, you'd need 4 butts. 8 pounds/50 percent yield= 4 pounds per butt. 4 pounds x 1/4 pound serving is 16 servings per butt. 16 servings x 4 butts is 64 servings.
I always go with 1/2 pound per person, there's no greater embarrassment than running out of food. I always tell them this recommendation and if they want less that's on them.