Yes it is.
Plural: Heads of lettuce
Singular: Head of lettuce
8 for tomatoes and 4 lettuce heads
0.95
8-10 heads of lettuce depending on how much lettuce you use
One head of lettuce should feed 8-10 people, so for 50 people factor 5-6 heads of lettuce.
The correct spelling is "lettuce" (the leafy salad green).
a billion
28 = (70 / 5) * 2
Lettuce can come in bunches of leaf lettuce or heads of lettuce.
That is about 8 heads
13 pounds
The noun lettuce has varied usage in the plural. Traditionally the usage is "three heads of lettuce", but the alternative "three lettuces" has become very common during the last century. The collective noun is always "I like lettuce" or "this farmer grows lettuce".
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.