There are 3 approaches to size; individual pies, double crust pies (regular size) and chafing dish pans

There are some decisions about crust:
2 layer pie crust
top only pie crust
top only puff pastry (my favorite)
biscuits

Then a few protein or ingredient decisions:
with or without cheese
with or without tofu (if you use tofu, it is good to fry it up in little chunks before mixing in.)

All this before you get to the filling!

For 100 people you need about 4 1/2 gallons of sauce or gravy.

Here is a sample recipe-
2 cups oil
3 Cups flour
12 quarts flavorful veggie stock
16 Cups diced onion ( or pearl onions)
16 quarts any combination of: diced carrot, celery, fennel, mushrooms
8-10 lb diced blanched yukon gold potatoes (cooked)
1 gallon fresh peas (raw) or asparagus in 1 inch pieces
16 lb tofu, seared with salt and pepper (this is where you could use meats, also)
10 tablespoons whole grain mustard (DO NOT OMIT)
5 cups dry white wine
fresh thyme 1/2 cup
fresh tarragon 1/2 cup (DO NOT OMIT)
1/4 cup kosher salt
1 cup nutritional yeast (optional)
white pepper to taste
16 pounds vegan puff pastry dough

Instructions:
400F oven
Blanch potatoes. Saute leeks or onions in 3 oil, for 5 minutes on med high heat until translucent. Add carrots, celery, fennel and/or mushrooms salt and pepper. Turn heat to medium low and saute until carrots are al dente, about 12 minutes. Splash with wine, and cook on med heat until it evaporates, about 5 minutes.

Stir in flour and nutritional yeast and and cook one minute while stirring. Add stock and mustard and stir until it just comes to a boil and thickens. Turn heat off. Fold in cooked potatoes, fresh peas or asparagus, and seared tofu (or cooked chicken). Add fresh tarragon, thyme and white pepper.