IMO - and I've cooked for many a crowd, whatever final number you think is about right, add an extra 25%. If you think 3 butts is enough, cook 4. If you think 5 racks of ribs is enough, make 7.
One of the most embarrassing moments of my life - church BBQ. I made pulled pork and brisket. The line went though and about 3/4 of the way through I noticed the brisket pan was empty. Pastor and I were standing there yapping and he said 'dang, I was looking forward to some of that brisket'.
That night I started another brisket after I got home. The next afternoon I took most of it to him. I had to have a couple of lbs for me....
Leftovers are better than people standing around with that sad look on their face. Pork is cheap. Dignity is priceless. Embarrassment cuts to the bone.
3 ribs per person is ok until you get cousin Jimmy who puts away 3/4 of a rack and turns his nose up at coleslaw and tater salad. If I host, and Jimmy wants to eat 2lbs of meat, Jimmy can eat his fill. I'm not going to stand there and say "hey Jimmy, back off on the ribs"