I'm having a graduation party for my son in late June, hopefully after things open up from quarantine. So far, we've invited 275 guests, but I have no idea how many will actually come due to social-distancing. I think I should plan for around 200?

4 open houses are occurring on the same day. Mine is 4-8pm.

It is a chips and dips menu. Here are the estimates that I tried to figure out from your site:

Warm - Cheesy bean dip – ?
Warm - Spinach/Artichoke dip – ?
Warm - Buffalo Chicken Dip – ?
Tortilla Chips – 16 lbs
Pita Chips – 10 lbs (I assume that homemade is cheaper?)

Warm – Pizza (Marinara) Sauce Dip - ?
Bread Sticks - ?
(Or would sliced bread French bread be cheaper – 10lbs?)

Roasted Red Pepper Hummus – 2 gallons
Ranch Dip – 1 gallon
Baby Carrots – 10 lbs
Celery - 10 lbs

Watermelon slices – 10 lbs
Either grape clusters (10 lbs) OR strawberries (10 lbs)

Cheese Cubes – 8 lbs
Olives – 3 lbs

Cake – (8) 9x13 pans (Not sure on this as I usually see people have leftover cake.)
Maybe a candy like M&Ms in smaller paper soufflé cups- ?

Tea – 10 gallons
Lemonade – 10 gallons
Water – 5 gallons

Here are my prep and serving thoughts:

Prepare many dips ahead and freeze in foil pans. (Which ones would freeze the best?) Thaw before event, then warm dips in oven. Place on a rectangular electric food warmer tray for serving. Serve hummus room temp and Ranch dip cold. Does this plan sound good?

Should I have soufflé cups for dips, and if so, what size? I will get forks, but what about knives/spoons?

Cheesy bean dip – 4.5 cups
Spinach/Artichoke dip – 4 cups
Buffalo Chicken Dip - 4 cups

Because of the timing, you may have folks hungry, which means looking for dinner, when they arrive, And these are teens, which eat at least 10% more than adults, so these quantities are going to look like a lot.

Also, with all the hot dips, I am concerned about oven space. Strongly suggest you convert 2 dips to cold or at least crockpot, esp if you do meatballs.

Amounts
Dips at least 1 quart per 10 people, total volume with multiple flavors, so 20 quarts= 5 gallons for 200. Divide by your local preferences, this varies from region to region Since your recipes make about a quart, you are looking at 20 recipes.

At least 8 pounds chips per 100

Meatballs, cocktail size, 15-18 pounds per 100, 3 to 4 quarts sauce per 100.

Consider freezing dips flat in baggies. Easier to store, thaw.

Warm – Pizza (Marinara) Sauce Dip - ?
Bread Sticks - ?
(Or would sliced bread French bread be cheaper – 10lbs?)
I would not include unless on the meatballs.

Roasted Red Pepper Hummus – 2 gallons
Ranch Dip – 1 gallon
About right
Baby Carrots – 10 lbs
Celery - 10 lbs
2 ounces veg per person, or a least 1 pounds per 10 ready to eat. Consider having cherry tomatoes, bell pepper strips or mni bell peppers for color- you would have celery left over.

Watermelon slices – 10 lbs
Either grape clusters (10 lbs) OR strawberries (10 lbs)
Watermelon is very short, I figure at least 1/3 pound per person. Take a serous look at the fruit tray page

Cheese Cubes – 8 lbs. This is less than 100 ounces. I usually figure 12-16 pounds per 100...
Olives – 3 lbs- I usually allow 1 gallon per 100. These cost about $12 per gallon at restaurant supply

Cake – (8) 9x13 pans (Not sure on this as I usually see people have leftover cake.) A 9x13 cake cuts at most 15 pieces. Consider adding bars, cookies, or more cake. You want about 2 sweets per person
Maybe a candy like M&Ms in smaller paper soufflé cups- ? Put hem n he cookies and bars?

Tea – 10 gallons, part unsweetened
Lemonade – 10 gallons
Water – 5 gallons
Consider adding a coffee urn? Or a least a drip set up?

Ice at least 1/ pound per person.