One potato, two potato....

I came across a recipe for Smashed Potatoes on the Cooks Illustrated website using red bliss potatoes. I used miniature red bliss potatoes, and a 2 pound bag made enough of the recipe for at least 4 people. I refrigerated the leftovers, and I have to say the chilled result was one of the best cold potato salads I have ever tasted. The dressing was moist, and the potatoes were not at all mealy or dry. It was one of those happy accidents that will have me making the recipe again, but for the dual purpose of having a great potato salad as well. So, you can double or triple this recipe as a hot dish and count on having potato salad the following day. The secret ingredient is cream cheese, mixed with melted unsalted butter.

Especially since we don't know quantity of potatoes you are using now, use trial & error.
Make your regular potato salad (keeping notes on potato volume).
Then, regular less a noticeable amount (say, 1/5 volume). See if you have leftovers with your crowd. If no, stop there and use that measure for futures.
If yes leftovers, rinse & repeat until you reach best volume for your family, friends, occasions. :)

Funny, I find most recipes are better the next day. I will agree that when I usually make too much, the pile of whole potatoes never looks as much as they do once cooked and cut up.

I estimate by the pound. Potato salad is usually served in a situation where you don't really know how much (or little) is going to be eaten. I would always err on the side of "too much." Then again, I have no aversion to leftover potato salad.