When preparing a tossed salad for a large group of people, the rule is 3 ounces per person, according to the Seattle Times, so 300 ounces of salad would be needed to serve 100 people. That breaks down in different ways, depending on the specific type of greens being used.
For example, Caesar salad for 100 diners requires about 20 pounds, or 15 heads, of romaine lettuce. Spinach, bought in bunches, has a weight comparable to romaine, so a similar amount is needed. However, for a salad of mixed field greens, the vegetables are typically purchased already combined, so roughly 14 pounds of mesclun feeds 100.