While we don't have any scalloped potato recipes in large quantities on our web site, I recommend the books Food for Fifty by Mary K. Molt, the book Volume Food Preparation By Thomas J. Hickey or the publications from the Culinary Institute of America which all have recipes in larger quantities. For fifty servings, most recommend about 16 to twenty pounds of potatoes peeled and sliced from 1/16^th inch thin to 1/8^th inch to no more than ¼ inch thick for a serving size of of 6-8 ounces.  For 75 servings, increase by half or 24 to 30 pounds of potatoes. You’ll need to cover the potatoes with a roux made from butter, flour and milk or cream, adding warm liquid to the potatoes and then add shredded cheese on top. Or, add a layer in the middle just like lasagna of a soft or shredded cheese.

Find a couple recipes below that can be scaled up, using the ideas for ingredients listed.

This consumer recipe (which I found online) for au gratin potatoes clearly shows the steps visually. It gives some options such as cutting the potatoes with skin on into slices and then quartering (saves time, adds flavor) and adds garlic for flavor and a liquid to help simmer the potatoes, then finishes off with shredded cheese which could be done at the last minute and then melted. It's from a wonderful blog, The Pioneer Woman. The quantity is much smaller, but you get a sense of home the dish should look at various stages.