"You WILL run out of ribs, espically if you are doing BB. I would get at least 8 racks...if the kids eat ribs to then maybe 10 racks should be in order."
Ummmm, you may not want to do a full rack for everyone who attends !!!
A good rule of thumb for BB, is that if you have a mixed crowd ( men, women, children ) then 2 full racks will feed 3 eaters.
Good Luck and Have Fun !!!