Well, last weekend, i made 2 5lb. butts. Had 5 people over, the 6 of us at 1 whole 5lb butt and a whole chicken! And there were 2 girls in the group! But then again, i'm a HUGE eater, i don't know how given that i'm 6'2" weight 185!
We also had slaw and other stuff, but that goes to show that it really depends how much people eat. I say make as much as possibly and freeze the leftovers! There is never too much pork butt!