Here is a recipe for 200:

chicken broth 24 quarts
celery, chopped fresh 2 gallons, 1 quart- about 11 pounds raw
onion or shallot, fresh, chopped 2 1/4 pounds/ 6 1/2 cups
salt -1/4 cup
pepper(white if you have it) 2 tablespoons

milk 2 gallons, 1 quart (may use part cream or half and half)
chicken broth 6 1/2 quarts - a bottle of sherry may be substituted for part of the broth.
flour 7 1/2 quarts

chicken, cooked, diced, 40 pounds (see note)
red bell pepper (canned pimento) 3 1/2 cups
green bell pepper 6 1/2 cups
OR you can use mushrooms instead of or in addition to the peppers.

Prepare the vegetable base:
Bring the broth celery, onions salt and pepper to a boil, immediately reduce heat and cover. Simmer about 10 miutes until tender. Stir milk into vegetable base.

Thicken sauce:
Mix chicken broth and flour until completely smooth, stir into vegetable base, stirring constantly. Cover, reduce heat, simmer at least 10 minutes, stirring frequently to avoid sticking.

Add meat and peppers:
Stir in chicken, peppers. Simmer 2-5 minutes until peppers are tender. Must reach 165 degrees after the chicken is added.

Chicken note: Traditionally, this is breast. If using whole chickens 80 pounds. If you buy the chicken raw, simmer it is the vegetable base to cook it, before thickening the sauce- this gives tender, juicy chicken. It is more traditional to cut it up after it is simmered, rather than cutting raw chunks- the texture is a little different.