How much dry noodles for macaroni salad for 25 people?

That probably depends a lot on the appetite of your eaters, but as a side dish about 1/4 to 1/3 cup of dry macaroni per person should do nicely with an entree and a vegetable. You can always adjust this amount accordingly for large appetites or light eaters, but if you end up with leftovers, they are easy to make into a cold or hot salad for lunch the next day. (Sometimes, I purposely make more macaroni than I need just to have some leftover for this purpose.) Just add some finely chopped green onion, leftover chicken or light water-packed tuna, a little parmesan cheese, and some fresh ground black pepper for a very quick and tasty meal. (If you use tuna, you won't need to add any salt; the tuna and the parmesan cheese have plenty.) Finely chopped celery or leftover broccoli bits make nice additions, too. If the salad turns out too dry for your tastes or it just needs a little 'zing', then add a touch of your favorite Italian dressing, or splashes of Apple cider vinegar and a good quality olive oil. For a more creamy taste and texture, a little mayonnaise makes a nice substitution for the Italian dressing.

If you're creating a casserole or main dish from the macaroni, then I would still use about 1/4 to 1/3 cup dry per person. You can always compare this amount to similar recipes in your cookbooks or to recipes online. For example, if I found a casserole in a cookbook with similar construction and ingredients (same portion of meat and cheeses for example) that called for 2 cups dry macaroni and served 6, then I would use about the same amount in my casserole.