Like UTEE, I usually figure on 1/3 -1/2lb. cooked meat per person for fajitas depending upon who I am feeding, what other food is involved and what time of day. For 30 people that means about 10-15 lbs of cooked meat. For flank steak and chicken breasts you can count on about 30% cooking shrinkage, so you are going to need about 14-21 lbs of raw meat to start with.If you use pre-marinated meat, the marinade the meat is packaged in does act somewhat like a brine and will reduce the cooking shrinkage, but sometimes there is considerable liquid inside the package that is included in the total weight so the it's probably a wash in the end. Marinating your own is easy, but cooking for a group is not the best time to experiment. If you have a proven recipe and procedure of your own, however, I would go with that.
See AlsoSteak FajitasSorry, but fajita sizzle isn't real. TikTokers have discovered restaurants just add water or a sauce to make it appear like the dish is cooking.Steak Fajitas RecipeThe Best Way to Cut An Onion Into Strips for Fajitas | Front Range FedIf it were me I would get about 10lbs each of flank and chicken breasts (20lbs total) because they make great leftovers if you have a surplus, but if you run out of meat that is one of the things that people will remember about the party.
As far as reheating goes, fajitas that are cooked (not overcooked) in advance reheat well in a foil covered pan in the oven (200-250). A chafing dish, if you have access to one, works great for keeping them warm and for serving.
If you happen to overcook them and they get dried out, just make up a chimichurri sauce and mix it up with the meat.
Best wishes.
