Do you serve a sit down 3 course menu or a buffet. If a buffet how many more dishes beside the potato salad. Based on that we as Chefs will only know how many Grams per person to serve and it would not matter if you serve 1000 guests.
Personally I would go with 50 lbs of potatoes ap, but factors to keep in mind are that your yield percentage on potatoes is 80 percent. And whether you serving the salad as a side serving, or on a buffet .
As many have stated, it really depends on what else you are serving. Is this the only side? How many meat offerings? Buns with the meats or not? If this is being offered along with other salads-sides, then 4oz per person is plenty. That is 4 oz, by weight of salad, not potatoes. So you need to do some figuring. Are the potatoes being peeled or left skin on? How much stuff are you adding to the potatoes? For 80 people 4oz of salad would equal 20 pounds, so I usually would use 17-18 pounds of potatoes, peeled or 15-17 pounds unpeeled.