A 13 pound (bone out) to 16 pound ham (bone in) will feed fifteen people. This will depend on how much each guest will eat along with any sides provided. Typically, one person will eat between a half a pound of bone out or 3/4 pound of bone in ham.
More or less ham may be needed, depending on a few factors. Guests who are hearty eaters will require more ham, while guests who are light eaters will require less ham. If there are more well-loved side dishes for the guests to choose from, less ham will be needed. If the ham is served buffet style, less ham will also be needed.