While the amount of prime rib you need will always depend on your guests’ appetites and what else you’re serving with it, a good rule is to buy one rib for every two people, according to the blogger behind Natasha’s Kitchen. For example, an 8 to 10 pound bone-in prime rib roast contains about four to five ribs, so it will feed eight to 10 people. She adds that if you have a larger spread of side dishes, you could also get away with using one rib per three people.
Southern Living echoes this sentiment, explaining that you should expect the average dinner guest to eat anywhere from half a pound to a full pound of prime rib based on the menu. However, the culinary site says if you aren’t sure, to always err on the side of ordering too much. After all, there are plenty of ways to enjoy your leftover steak the next day!