There's another thread on Q-Talk right now asking basically the same thing. I think the consensus is about 1/2 lb per person is a good estimate.
See here:
http://www.bbq-brethren.com/forum/sh...ad.php?t=88540
I squeezed 5 butts on an 18" WSM this summer. 3 smaller ones on one rack and 2 on another. I'd think that 4 -- 8-10 lb'ers should be plenty. Remember, post cooked weight is always gonna be less that pre-cooked.
Good luck!