I have a recipe for butternut squash soup that calls for 2 potatoes which will be pureed in the end. I only have potato flakes. How much of the flakes do I use to equal the 2 potatoes?
I would aim for the amount of flakes that would make 2 cups of finished mash. Add 1/2 to 2/3 of it and see where you are in terms of thickness of the soup. If the soup is still thin add more.
This also depends on what potato the flakes are made from, any additives to the flakes, etc.
Usually when a recipe is not specific with an agricultural product I assume I have a lot of margin for error. Cooking requires a lot of experimenting, but using the flakes may actually make your life quite a bit easier here as you can add as much or as little as you want "to taste."
Looking at the directions for a random brand of potato flakes, two portions of mash comes to 1 cup. To make this you need 1 cup of liquid (water and milk) and 2/3 cup flakes. Obviously the measurements in these directions are extremely rough since this must come to more than 1 cup. No matter; I'm sure you are not expecting anything more than a ball-park figure.
A six ounce potato makes roughly 1/2 cup of mash. So that's about 2 tbsp (~30ml) of flakes for every ounce of potato (again these are rough figures).
Potatoes come in all sizes, but a six ounce potato is about average for medium sized potatoes. If you have no other information you could just say that one portion of mash made from flakes is equal to one potato. In that case you need about 2/3 cups of flakes.
As another answer says, the potato will act as a thickener, so you may just as well add mash until you get the consistency you want.