Craig,
It was 8 lbs of cooked bbq. This situation was a bit different because I actually bought the bbq at a local joint.
My plan was to smoke a 9.5 lb butt,grill a dozen hamburgers and a pack of hot dogs. I wasn't sure how the quests would like my bbq so I wanted to do some other things. I knew I'd have leftovers but I have a Foodsaver and don't have a problem getting folks to take stuff home.
Temps well over 100 degrees changed my plans. I'd done some ribs a few days earlier when temp was 111 and took a day or two to recover. When I cancelled my plans and decided to just go with store bought pork I estimated about 6 ounces (cooked) per adult. As I mentioned, I overestimated but we did have alot of sides and I think the weather reduced consumption. No problem though, bbq doesn't go to waste around here.
Using my estimate you'd need to buy about 14-15 lbs of butt for 20 adults, assuming a 50 percent shrinkage/trim rate and 6 ounces per adult. You'd have to have some pretty big eaters to run out. You might could get by with as little as 10 lbs of precooked, assuming the same shrinkage rate and target of 4 ounces of cooked (decent size sandwich) per person.
P.S. Just noticed Dave's forumula. That's what I use for estimation. I've never cooked brisket so I don't know about it.