You want about 20 pounds potatoes, depends on size if you bake them, 1 each plus a few extra, see the taco/potato bar page.
You need 1 pound boneless, skinless, or 2 pounds whole for each 3 people.
4 pounds cabbage, 3-4 romaine, 2 quarts dressing.
You need to add rolls, and to make sure there are no vegetarians, or get a pack of frozen unchicken cutlets for them.