For just 10, smoke one average sized butt.
For 20, smoke two average sized butts. You go to the trouble, you might as well have leftovers, and the meat is cheap. (By average, I mean an eight pounder, which you can easily fit four of in a wsm.)
If trying to figure for a LARGE event, then I do the math: 10 oz of UNcooked pork butt per person. In other words, you only NEED a 6.25 lb. butt for 10 people, and two pork butts that add up to 12.5 lb for 20. Personally, I'd just cook two average butts, either way. BBQ leftovers always gets eaten up around here....bbq nachos, bbq stuffed potatos, bbq omelettes, bbq sandwiches, bbq pizza, wraps...
I always figure 1 pound uncooked per person for the smaller groups. I have never had a problem with having too much left over. I am usually lucky to have a little left to freeze for apps or something later.
While an old thread, this page is coming up often in Google search results, so I will add one more thing. I have a Pulled Pork Butt calculator that can help answer the question of how many pounds of uncooked meat are needed to produce a certain number of portions. You'll find it here:
I just did two butts totaling right about 18 pounds. That fed twelve, we had some cancel due to inclement weather.
I really only used one butt, a lot of the rest was sent home in “To go” packages. And, I still had probably three pounds to freeze!
I just looked, this thread is eleven years old!