"Little improvement" vs. "Little improvements"?

If the term Little improvement refers to the stagnance of progress, and Little improvements refer to a number of small additions, what is the plural of many stagnances of progress?

As in, "You have very little improvement. All of you have __ __!"

Because little improvement, with no article, is an uncountable mass noun, it has no plural and stays the same when shared by several people:

Caesar and Pompeius have shown little improvement.

It is similar to other abstract words, like love and compassion:

Cleopatra and Agrippina had little compassion for those who could use it.

But they gave much love to those they could use.

In your first example, improvement is not some countable thing. So if a lot of people each show little improvement, then collectively they all show little improvement. (Also note that in that sense of improvement, you normally show, exhibit, or achieve it; rather than have it.)