The amount of lettuce you need for 100 people depends on how it is being used. A good way to estimate is by having approximately a cup serving for each person, and then include extra just in case.
For 100 people or more, you need 15 heads of romaine lettuce to make a Caesar salad, 8 pounds of leaf lettuce for a garden salad or 12 pounds of lettuce for a tossed salad. The main difference in how much lettuce is needed is what other ingredients it is being combined with. Some salads have more ingredients and toppings; therefore, a serving for that person ends up needing less lettuce.