13 pounds
28 = (70 / 5) * 2
You don't mention whether this is for salad or for sandwiches. For side salads, you would need 12 to 14 medium size heads of lettuce for 100 people.
This depends upon portion size, the type of lettuce you and how many other ingredients are in the salad. Generally you decide on a recipe and, according to how many portions it will serve, multiply the amount of lettuce per portion by 70.
If it's only a starter salad, I would use about 15 pounds romaine with 5 pounds other green veg.
One head of lettuce should feed 8-10 people, so for 50 people factor 5-6 heads of lettuce.
There are many choices in making a plain salad, but one suggestion might be lettuce with tomatoes only.
1 cut into little pieces
Each taco salad would have approximately 1-cup shredded lettuce. One large head of iceberg lettuce yields around 25-cups of shredded lettuce. For 150 taco salads, you would need 150-cups shredded lettuce or 6 large heads of iceberg lettuce.
8 for tomatoes and 4 lettuce heads
a billion
It depends on the size and if you're talking about a large dinner salad or petite side salad. A typical head of iceburg lettuce can easily be split into fours for a side cobb salad. However, for an entree, you might only get two servings out of a medium head.
1000 xD