Here is a recipe I found. I hope it works out for you. I never made potato salad for that many people. Good Luck.
20 lbs. boiled potatoes
1 cup diced onion
5 cups celery, chopped
3 dozen hard boiled eggs, sliced
salt and pepper to taste
2 quarts miracle whip
2 quarts of heavy mayo
1 3/4 cup sugar
2/3 cup mustard