Hmm, that's a good question, one I'm probably not qualified to answer. The only times I cook for large quantities of people we always have several other main dishes so I'm not concerned with making enough. I always have leftovers as well.
That being said, I'd say 16 pounds would probably be adequate. That allows a pound per person before cooking. If you want you could do more, and freeze/give away the leftovers. I personally like to play it safe and always try to have more than unnecessary. 16 pounds will probably leave you with little to no left overs.
As far as cooking, you could probably get away with cooking at 300 degrees, maybe want to mop it at that heat. Certainly 250 or 275 you could do. I don't know how much time you're going to save though, maybe 3 hours if you're lucky. If you're dealing with 8 pound butts you're still probably looking at an absolute minimum of eight hours even at high heat, and most likely longer. I could be wrong, I don't have much experience cooking higher than 250-275.
I'd suggest doing it overnight. If you've never done it before don't worry about it, my second smoke ever was overnight. I'd probably allow about 1 3/4 hours per pound. If you're having a party you DO NOT want people waiting for hours after they thought they were going to eat. Better to finish early, then double wrap in foil and throw in a cooler. I've held meet like this for 4 hours and pulling it out of the cooler it's been as piping hot as if I just took it off the smoker.
For overnighters, I usually give it about an hour and a half to two hours after starting the smoker to make sure temperatures have stabilized, then I go to bed. Then I usually wake up early (but not too early) at approx. the expected half time to check temps and make sure everything is running ok.
Plus, in case something goes wrong (although it shouldn't) you should have enough warning to put a backup plan into place.
When I do it I allow about 1 1/2 to 1 3/4 hours per pound and plan on them finishing about two hours prior to when I want them done. Should the meat finish fast I can always hold it, and I should have a decent window to finish if the meat takes longer than expected to cook.
