It can sometimes be rather difficult to know how much meat you should buy when you are cooking for a large group. Prime rib is pretty big too, which can make it even more difficult to gauge how much you are going to need. There are some general guidelines that you can follow, however. It’s best to count two people for each rib. This means a three rib roast should feed six. As such, if you wanted to feed an extra two people on top of this then you would need a four rib roast. You will usually find that a two rib roast weighs roughly 4 pounds.
If you are still having trouble figuring out how much meat you are going to need, it’s certainly in your best interest to ask the butcher for their advice.