I have always budgeted by the bone - like Mdboatbum. I start at one bone per person. that gives you a 1.5-2" thick piece of meat - the entire diameter of the rib. That is very large, only the most dedicated finish one of these off. Then I look at who are my guests. Many women and some of the guys are happier with a half portion. I make those adjustments, then add 1-2 bones to be sure I have plenty (I'd rather have leftovers, then go short).
One of the common preparations for prime rib is to cut the bone 99% off the meat, then tie it back with butchers twine. If you do this, then it is easy to separate the bones after cooking and cut a bone slice portion in half - or take 2-3 bones off and slice thinner portions.
I never smoke anything smaller than 5 bones - you can get 7 and 9 bone ribs fairly easily. Last one I did, I had a 5-bone rib and two men and two women, but everyone loved prime rib. The 5th bone was left over, but 4 bones disappeared!