If they're really, REALLY good ribs, probably about 100.
Where I come from, 3 bones per person isnt hardly a snack. Mind you, we're EATERS
'round here. Seriously though, if it's more a social gathering with a LOT of food, 3 bones each is ok. If they're there to eat BBQ, Ribs, etc., and you make really scrumptious (my wifes word) ribs, you'll need a rack or two each. I made some 25+- racks last time and 30+- people showed up. The ribs were gone in about 30 minutes (yes, all 25 racks). Luckily I'd made another 6 9# butts to hold off the real eaters!
What can I say... I say "we're having a BBQ" and they literally fly in from Denver.
Hance - Lake Dogs Cooking Team - MiM/MBN/GBA CBJ and comp cook
Lake Sinclair, GA (strategically about an hour from darn near anywhere)
Started competing in chili cookoffs back in the 1990's and have competed in more than I care to count. I became a CBJ in MiM in 2005, then MBN and in GBA in 2010. I've probably judged 130+- BBQ comps (sanctioned and unsanctioned) over this time. That said, I really enjoy competing more than I enjoy judging, and hope to get back to doing 4 or 5 a year in the near future.