Adiochiro3...nice write up and pics.
Dan,
You've received good advice so just a few more thoughts from a cook I did in February for the same size crowd.
Get untrimmed whole butt. I get cryovac 2 butt packs. Bone-in or boneless is your choice but like others mentioned I use bone-in after i did a side-by-side cook of both and the bone-in won the taste test at a party we had. Try to get packs where the butts appear about the same size. It's not a big deal but does make cooking times more consistent if the roasts weigh the same amount. If not then just monitor the smaller one more closely. Roasts will come typically around 8-9 pounds and I generally assume a 40% yield with bone-in. Cooking time will depend on the details of your actual cook but figure around 2 hours / pound of your largest roast before trimming. The cook I did took right around 16 hours @ 225 to an internal meet temp of 195. Flip the meat once around half way. When they hit your final temp pull them, as mentioned, and tightly foil for the cooler sit on a few layers of towels for an hour. As Adiochiro3 said they can sit in the cooler for up to 3-4 hours. Preheat the cooler by putting a pan of boiling water in it 20 minutes before your butts are done. Note you may see your meet plateau at around 160-170 during the cook...don't panic it's just everything getting nice and tender.
On the serving size question we did the typical sides of beans, slaw, salads, etc and 4 butts (2 cryovacs) took care of right at 40 with left overs.
Let us know how it turns out.