My favorite potato for this recipe is Golden potatoes. They naturally have a creamy texture and rich, buttery flavor. They are less likely to taste watery or have a gummy texture (because they have less starch).
It's okay if the raw potato chunks are slightly bigger than 1-inch. Just try to keep all the pieces about the same size.
Plan for ½ a pound of raw potatoes per person. So if you're feeding 4 people, use 2 pounds of potatoes. If you're feeding 10 people, use 5 pounds of potatoes.
No matter the amount of potatoes (2 to 5 pounds), use 1 cup of water.
If the cooking time ends and the potatoes aren't soft enough to be very easily pierced with a fork, then just re-seal the lid and cook the potatoes for 1 to 2 more minutes.
The Instant Pot has a "keep warm" setting but if mashed potatoes stay in the pot too long they will start to stick and burn to the bottom. If you're keeping the mashed potatoes warm, make sure to stir them frequently and add milk if they start to dry out.
This recipe was tested in a 6-quart Instant Pot.