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 everywhere)