This was my dilemma this weekend, when I had to make a serves-50 macaroni for an event. I had a Joy of Cooking recipe for "Macaroni & cheese for a crowd", but when I read it, it only filled 2 9x9 pans and made 16 servings. I was cooking for a different-sized "crowd".
4 pounds elbow macaroni
7 cups milk
2 pounds colby cheese, shredded (I couldn't find colby, so used colby-jack)
2 pounds sharp cheddar cheese, shredded
1/2 cup butter
3/4 cup flour
2 cups more milk
1 teaspoon salt
1/2 teaspoon white pepper
1 cup seasoned bread crumbs
1 small box Ritz crackers, crumbled (I didn't know what a small box was. I used 2 1/2 stacks, if that makes sense. It was plenty.)
and mixed with
1/2 cup melted butter
1 lg. (18"x12"x3") foil roasting pan
Make a roux by melting butter, add flour and stir, then cook over low heat for 2 to 3 minutes while stirring constantly. Set aside to cool.
Cook macaroni according to package directions. (This is not as easy as it sounds. The package directions only accounted for cooking half a package at a time- and I needed to cook 2 packages! I used my two largest pots to boil water & cook the macaroni, and all was well.) Drain and put into foil pan. While macaroni is cooking, heat 7 cups milk, add cheeses and cook over medium heat while stirring to melt cheese. Add salt and white pepper. Add roux and stir constantly until well thickened. (I got distracted and didn't stir this constantly, and the bottom burned. Be warned.) Add this to macaroni and mix well to coat macaroni. (Not easy, again, because the macaroni already just barely fits in the foil pan. Add a little at a time, stir in, add a little more, etc. Otherwise you'll have a mess. I got help with this from my obliging husband.) Pour in remaining 2 cups milk (Again, slowly) and sprinkle with bread crumbs or Ritz crumbs. You may prepare it ahead and bake it later. Bake for about 1/2 to 3/4 hour in 350 degree oven.