I make my potato salad from scratch and I usually make enough for 12 people. I use 15 medium potatoes, 8 eggs, and 2 small onions. If I were making for 100 people I would use 125 to 150 potatoes, and 2 dozen eggs, and 5 onions. I would then mix in enough mustard, salt, and mayonnaise to make it creamy and tasty. It would be best to make it in several smaller batches to make it easier to mix it all together.