Real authentic fajitas are made from marinated beef skirt steaks, not chicken, round steak, etc. That is not what most restaurants serve. The beef you select has to be tender enough to eat on its own, such as top sirloin.

For results like the popular restaurants, you MUST marinate the meat at least 4 hours, up to overnight. This marinade is DISCARDED when the meat is cooked.

You MUST sauce the veggies separately and add that sauce to the dish as a finishing sauce..

The best way to cook the meat is to grill the pieces, then thin slice them. It just is not the same if you pre-slice.

For a two fajita serving you need:

1 pound of lean boneless raw meat per 4 persons.
Equal weight of vegetables as meat: 1/3 onion, 1/3 green peppers, 1/3 red/ orange/ yellow peppers

Additional items:
tortillas- 4 per person, 2 as wraps and 2 as bread. Flour is traditional, but I include corn as well as flour. Nowadays, you can also offer whole romaine or large iceberg lettuce leaves, as carb-conscious folks will make lettuce wraps.
grated cheese- 1/4 cup=1 ounce per person:
For 100, 2 tbsp=3 lb 2 oz= 3 1/4 quarts; I would double
guacamole- 1/4 cup per person
pico de gallo/ salsa- 4 tablespoons per person is 7 pounds or 6 1/2 quarts for 100
tomatoes, diced fresh- 2-4 tablespoons per person; per 100 people, 4 tablespoons is 13 pounds
shredded lettuce- 1 to 2 ounces per person
sour cream- 2 tablespoons per person is 6 1/2 pounds or about 13 cups per 100

Usual to serve with whole beans such as borracho beans and rice.

Here is a pretty good recipe:

Meat marinade for 1 pound meat:

1/4 cup lime juice

1/3 cup water

2 tablespoons olive oil

4 garlic cloves, crushed

2 teaspoons soy sauce

1 teaspoon salt

1/2 teaspoon Liquid Smoke (DO NOT OMIT!)

1/2 teaspoon cayenne pepper

1/4 teaspoon black pepper

Combine lime juice, water, oil, garlic, soy sauce, salt, Liquid Smoke, cayenne pepper, black pepper and either the sirloin or the chicken in a container, cover and refrigerate at least four hours or overnight.

Remove from marinade and discard marinade. Grill meat over medium flame 4 or 5 minutes on each side. Cut meat into thin strips. Set aside and keep warm- save juices

Finishing sauce and vegetables:

2 tablespoons water
1 teaspoon soy sauce
1/2 teaspoon lime juice
scant black pepper
1 tablespoon olive oil
1 large Spanish onion, sliced thin
1/2 green pepper, sliced thin
1/2 red pepper, sliced thin
1/2 yellow pepper, sliced thin

Combine the 2 tablespoons water, soy sauce, lime juice, salt, pepper and olive oil. Set aside.

Cook onion and peppers in oil until brown. Remove from heat. Pour reserved finishing sauce mixture over onions and peppers.