A standard loaf makes about 8 sandwiches, so plan on 5-6 loaves, 6 if it will be 30+. This allows some variety, such as sourdough or potato, marble rye, couple of wheat types. Maybe corn tortillas for some quesadillas?

1 pound sliced cheese per 5 (at least 3 1-ounce slices per person- and do get sliced, not shredded), nice melty ones such as cheddars, jack, havarti, muenster, swiss. I don't choose mozzarella or provolone because of their so mild flavor. If it is not an adventurous crowd, do get 1 package of American.

Add-ins:
Several pounds very crisp bacon (made ahead)
thin sliced tomatoes-or julienne dried tomatoes in oil
deli sliced meats- about 1 pound each ham, turkey, beef; maybe also 3-4 quarts of tuna salad
green things such as spinach
potato chips
maybe nut butter, sliced black olives, thin sliced red onions or green onions, or very thin sliced apple slices dipped in fruit fresh or orange juice to keep from browning?

plenty of soft butter and very good mayonnaise
mild and spicy mustard

You might enjoy these off-site articles-

www.yourhomebasedmom.com/grilled-cheese-party/

www.oprah.com/oprahshow/How-to-Make-Your-Own-Grilled-Cheese-Bar