A good rule of thumb when making potato salad for 50 people is to assume that each person will eat half of a potato. Using this logic, you will want to change the recipe, adding ingredients, to equal the amount needed for 25 potatoes.
There are 16 cups in a gallon. So a gallon of potatoes salad would serve 16 people at a cup each. Since not everyone will eat a full cup a gallon should serve 20 people. So 50 people you will need around 2 -2 1/2 gallons for 50 people.