You need about 3 quarts for 100 if you are dressing the salad, but a gallon if you have several that the people will select and pour on themselves.
Here are some options.

Basic Creamy Italian Dressing
Make it a day ahead and let it blend in the refrigerator.
9 cups mayo
1 cup milk
1 cup white wine vinegar
2 tablespoons sugar
4 teaspoons fresh crushed garlic OR 2 teaspoons garlic powder
3 tablespoons Italian seasoning OR up to 1/2 cup fresh herbs (keeps better with dry)

Olive Garden-style Italian Salad Dressing
3 quarts
Make it a day ahead and let it blend in the refrigerator. It contains thickeners that keep it from separating easily.

4 cups white or red wine vinegar
2 2/3 cup water
2 2/3 cup vegetable oil or olive oil
2 cups light corn syrup
1 1/4 cup grated Romano cheese OR Romano Parm blend
1 cup dry pectin (comes in a box, used to thicken jelly)
1 cup beaten egg or egg substitue (5 eggs)- USE PASTEURIZED EGGS!
3 tablespoons salt
3 tablespoons lemon juice
4 teaspoons minced fresh garlic or 2 tsp. garlic powder
3 tablespoons dry Italian seasoning
1 teaspoons of crushed red pepper flakes, or to taste

Combine all of the ingredients with a mixer on medium speed for 30 seconds -Or- in a blender on low speed for 30 seconds -Or- use a wand mixer on low speed for 30 seconds. Thickens as it chills.

George Diamond's-style pink "French dressing"
Make it a day ahead and let it blend in the refrigerator.
Makes: 120 servings

3 (10-ounce) can condensed tomato soup
2 cups oil
1 1/2 cups white vinegar,
1 1/2 cups sugar
1 pound onion, peeled and grated
3 cloves garlic, peeled and halved
3 tablespoons dry mustard
3/4 teaspoon each: salt, ground black pepper