Is there a general rule for how many potatoes I need for mashed potatoes per person?
I think it's about 2 per person depending on the size of the potatoes.
It really depends on how large the potatoes are, and how much mash you want to serve - if you're talking the large russets, 1/2-1 each would probably be fine, with no leftovers. I usually err on the side of too much because they do seem to go quickly.
I go by weight: 2 lbs serves 4-6 people.