How much mashed potatoes for 5 people?

For holidays, I usually count 1 1/2 potatoes per person. Everyone gets as much as they want, and you have those heavenly leftovers. At a more sane time, I go with 1 per person.

Usually a half pound of uncooked potato per person is a good place to start. As long as you have space for leftovers, there is no harm in going over.