To feed 20 people, roughly 11 to 12 pounds of ham will be needed, depending on whether the dinner is buffet-style or portioned. A 14- or 15-pound whole ham will serve 24-28 guests if portioned, and a 9-pound half ham will serve 21-24 guests buffet style. It is a good idea to estimate higher, and better to have leftovers than not enough.
Counting small children should also be a consideration, as they eat much less than an adult. A 10-pound half ham will feed 18 portioned servings or 24-27 buffet servings. For 20 guests, slightly more ham will be needed.