In general, a single pasta serving is two ounces of dried pasta. For smaller pasta shapes that can fit in measuring cups, that is about one heaping cup of dry pasta.
To make 100 servings of pasta salad requires between 6 1/2 and 8 pounds of dry pasta; 2 tablespoons of salt; 1/2 teaspoon of pepper; 12 cups of diced celery; 6 cups of chopped green peppers, pickles or olives; 2 cups of minced onion; 1 cup of minced parsley; 1 1/2 to 3 dozen chopped hard-boiled eggs; and 1 1/2 to 3 quarts of salad dressing.
Assuming your pasta salad recipe includes plenty of veggies, cheese and maybe meat, one serving of pasta salad will probably include about 1/2 cup of pasta. A standard 1-pound box of pasta will generally yield about 8 cups of cooked pasta, so you can expect to feed about 16 people from one box.
The right amount of pasta per serving will also depend on how hungry your guests are. As a host, you should at least have an idea about the appetite of your visitors. For example, a team of teenage basketball players after a game will, of course, require more pasta than say, adults who just had a few drinks.