I would say you need two bones per person, plus, 1/3 pound of total meat of the other two. Figuring on that, you are looking at needing an additional 23 pounds of cooked product. Hence 2 packers, 4 medium sized butts, enough racks to give you 120 to 140 bones or so.
If it is plate service, that should work. For a buffet, just put the ribs at the end of the line. Otherwise folks load up on them and the count gets off.
Edit: the math
two 14 pound packers, 35% loss in cooking and trimming, 18 pounds finished meat.
four 8 pound butts, 40% loss in trimming and cooking, 19 pounds of meat
140 bones at 9 per rack=15 racks or so