An average sized serving of mashed potatoes is 1 cup, approximately one half a pound of potatoes. Five pounds should serve 10 people. Of course, if you have a mashed potato monster in your family, you may want to make more. My uncle always makes a huge mound of potatoes on his plate, shaping a nice hole in the center for several ladles of gravy. For him, maybe 2 cups of potatoes, or even more.