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Well you gotta break it down, what all goes into the mac n cheese? some recipes call for more ingredients than others meaning they will feed more people. Also depends on what else you are having including meats, other sides and desserts. And what is the size of a "hotel pan?"
As smoking said, depends on what else you're having as well as the diners. Obviously, football players are going to eat more than children.
That said, for sides I always figure about 4 oz per serving. You'll want some extra for seconds and left overs (if desired). So figure out the number of portions and do the math. Weigh you full pan of mac and cheese and divide by 4. That will give you the number of portions per pan.
Standard hotel pans with 4 inch sides typically hold 14-15 quarts. These are the roughly 12 X 20 X 4 inch steam table inserts. There are other heavy hotel pans with handles on the short sides for roasting that are larger and hold about 40-42 qts. These typically would not be used for mac, but the steam table insert would.