I'm having a salad bar party for 20,how much iceberg,romaine and spinach should I buy?

Thank You for your answers,I should have posted the menu:Romaine Lettuce Iceberg Lettuce Spinach Broccoli Cherry Tomatoes Beets Celery Red Onions Carrots Cucumbers Green & Black Olives Radishes Green Peas
Cauliflower Green Beans Avocados Black Beans Red Cabbage
Shrimp Chicken Breast Bacon
Deviled Eggs Colby Cheese Feta Cheese Cottage Cheese Croutons
Patio Salad
Grilled Vegetable Orzo Salad
Chinese Chicken Salad
Top Ramen Coleslaw
Tuna Salad
Ranch Italian Oil & Vinegar Thousand Island
Baked Potatoes Salt & Pepper Mashed Potato
Butter Sour Cream Mushrooms Chives Tortilla Chips
Baked Sweet Potato Cinnamon Sugar Pecan Topping Marshmallows Raisins
Crackers Garlic Bread Flat Bread
Ice Tea Lemonade Bottled Water
Mini Lemon Pies
I know it's a huge menu,but I love to have choices.

I would say at least 2 oz per person if it is not main dish. If main dish maybe 4 oz per person.

Are you doing several salads? Remember to make a few that are vegan (no bacon, eggs) and Gluten free.

One large presentation salad which would be in a large bowl could be a classic Cobb Salad. https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSsuLpv_eiPLl8jEGnztmCcGq2xV0UUA3i_5-YFre0ByD7qIpLMxg

Pay attention the colors and order of the rows of items...so you bland colors next to bright colors. Avacado, Blue Cheese, Tomato, Turkey, Eggs, Bacon.
Serve the brown derby cobb salad dressing in a gravy boat..and put some tongs for guest to grab a bit.

I would do 3 heads iceberg; 3 heads romaine or 6 hearts of romaine and 2 bunches of spinach or 20-24oz baby spinach