Fewer and Less :
When making a comparison between quantities we often have to make a choice between the words fewer and less. Generally, when we're talking about countable things, we use the word fewer; when we're talking about measurable quantities that we cannot count, we use the word less. "She had fewer chores, but she also had less energy."
The managers at our local Stop & Shop seem to have mastered this: they've changed the signs at the so-called express lanes from "Twelve Items or Less" to "Twelve Items or Fewer."
Whether that's an actual improvement, we'll leave up to you.
We do, however, definitely use less when referring to statistical or numerical expressions:
In these situations, it's possible to regard the quantities as sums of countable measures.