Soft and/or hard shell tacos depends on availability
Bell peppers (maybe, sliced or diced)
Maybe some chili peppers
Spanish rice like
Some kind of beans, if I can find refried would be the best if not Iíll make this mixture: beans, read onion, garlic and salt all put thru food processor
Charlie, I can see you aren't getting very far with the quantities aspect so I'll try to help here. People can perhaps offer you an alternate view of my suggestions then, but at least it gives you a starting point.
Now I would allow a tablespoon of meat per taco, and likewise of lettuce. Now as to the other toppings, it is highly unlikely in my (limited) experience that people are going to put more than three or four extra toppings on their taco at say a couple of teaspoons each.
As to the rice and beans, allow a couple of tablespoons per person.
Generally you would only cater enough of any one dish for 3/4 of the people. If this is a main meal, you will need to allow 3 or 4 tacos per man, 2 per woman, and 1 per child, plus some extras. These are guides only you understand and as Expatgirl said, have some extra bits on standby, unopened.
This isn't a perfect guide but as I said, others more experienced than I can now amend my guesses. But at least it is a starting point.