Less vs. fewer
“There are people who cringe at the grocery store when they see the sign '10 items or less,'” said Anne Curzan, a professor of English at the University of Michigan.
It seems as though the rule for less vs. fewer is becoming less clear.
She said, “The rule is that with nouns that are countable we should use fewer. And with nouns that we can’t count, such as water, we should use less.
“Ten items, clearly you can count them because there are ten, so it should be fewer. If you have money it would be less money, but fewer dollars.”
The principles are the same with amount vs. number, so amount for an uncountable noun, and number for a countable noun.
These lines get blurred a lot. Curzan said, “For example in academia, we often talk about writing an essay of five-hundred words or less.
“Now clearly you can count the words, ‘we just said five-hundred,’ but you don’t tend to say ‘five-hundred words or fewer.’ And then there’s the expression, ‘that’s one less thing to worry about.’ And you just don’t hear people say, ‘that’s one fewer thing to worry about.’”
For some speakers, Curzan says, fewer feels more formal, and less feels less formal.
“This has actually been happening for hundreds of years that less has been encroaching on the territory of fewer. One possibility is that less replaces fewer. Another is that the two live on synonymously, happily together,” she said.