120 guests including children - Amounts needed please for party catering
I know that people frown upon recipes on here, but in the book "Quantity Recipes: From Meals for Many" by the New York State College for Home Economics, 1945, they recommend a serving size of 3/4 cup, which for 50 to 55 people is:

9 lbs ground beef
2 lbs ground pork
1 1/2 cups onions, chopped
1 cup beef dripping or other fat
5 qts kidney beans (#10 can) or 1 3/4 lb dried
5 qts tomatoes (1 1/2 #10 cans)
2 1/2 TB chili powder
5 TB salt
1/2 tsp pepper
3 TB sugar
1/2 lb crackers, broken


I personally wouldn't use that recipe (1.5 cups of onions and 2.5TB chili for 11 lbs of meat?), but it should give you an idea of quantity.

You might be better off making a 'normal' batch of your chili recipe and measuring to see how much it makes, but possibly adjusting if it's significantly less than 3oz of meat per serving.

It's also worth noting that chili freezes really well.  If you have the freezer space, you might want to make some in advance and freeze it.  (I use 1qt deli containers, which would be 5 1/3 servings by their estimate).  Of course, I'd also freeze it without beans and add them back when reheating (or serve it separately as an option).