I'm no expert but I would prefer to cook 2 larger (9-10ish lb.) butts rather than 4 smaller ones if you can find them. I think the come out more moist. How many ribs do you think each person will eat. If an average rack has 12 bones x4 that's 48 bones. So you'll have a little less than two per person. If you think everyone will only eat 2 then you should be fine with 2 large butts, 4 racks and your 2 pans of sides. If it's me i'd make 5 or 6 racks because I always freak out about running out of food. Drives my wife nuts but keeps the people happy. Again i'm no expert.
"I can get a good look at a T-bone by sticking my head up a bull's a$$, but I'd rather take a butcher's word for it". - Tommy Callahan III
XL Big Green Egg, 22.5 Weber OTS, WSJ Gold/Mini WSM, 18" & 22" WSM, UDS "No.8"
Flyers Orange Thermapen
Napkin Killers BBQ