That depends how you make them. Potatoes on their own should not be considered protein, but rather a starchy carbohydrate. If you make your mashed potatoes with butter, a little milk and parmesan...
A quick rule of thumb that has served me well: one pound per person. For sixty people, that's sixty pounds.
If you are mashing red skins or Yukon golds, figure about a pound per person. There will...