But if the roast is part of a bigger spread with plenty of other food, you can plan on 1/2 to 3/4 pound of prime rib per person. Have your butcher order a roast in the weight range you need.
To buy prime rib from the butcher, ask for the dry-aged prime rib and specify that you want the meat to be "prime" (not "choice") and have a lot of marbling. The more marbling, the more flavorful ...