I allow about 2 1/2 ounces of croutons for one person, adding all the usual additions- lots of celery, onion, herbs, eggs, and at least one specialty item such as nuts, mushrooms, oysters, or cooked sausage. I always use turkey or chicken broth, except for the meatless pans I make for vegetarians. With 400 people, you will have some, and need to make sure you have some good food choices for them.