Speaking from experience, baby-back ribs go fast and everyone seems to like them, so plan on a couple for everyone, then chix thighs seem to go pretty well also, but not like ribs. Depending on the crowd, some folks always like PP and will have that with other meats when provided.
-4 slabs of BB ribs = 52 ribs, or just shy of 2/person: alternate (less $/lb and more bang for the buck): cut Spares (4 slabs) to St Louis Style and smoke the trimmings to cube-up and add to smoked beans);
-10 lbs of thighs will be approx 22-26 (depending on size) = just under 1/person;
-10 lbs of pork shoulder (~6.5lbs cooked) = roughly 3.5oz/person;
The above should give you leftovers if you have several sides like beans (especially loaded beans), slaw, tater salad, garden salad, maybe steamed veggies (smoked would be even better), etc. Don't forget the rolls...
Have a GREAT smoke and a fun party!!!