Nothing whips grammar purists into a frothing frenzy more than supposed errors in the use of “less” and “fewer.” The rule is pretty simple: use “less” with non-count nouns: less sugar; use “fewer: with count nouns: fewer cups of sugar. It’s an easy rule to follow.
But little complications crop up. One can have many problems—who doesn’t?—but suppose one of your many problems is an aging cat that’s costing you a bundle of money in vet bills, and the cat suddenly dies. Would you sigh with relief and say, “Well, that’s one fewer problem”? No, you would say, “That’s one less problem,” at least if English is your native language. Cats are countable, but would you say “Of all potential pets, I like cats the fewest”? No, that’s a different question–liking something more or less; most or least.
Certain expressions with less involving count nouns are quite idiomatic: Write an essay in 25 words or less. We have less than five miles to go. I believe that to most native ears the sign at the express line in the supermarket—10 Items or Less—sounds perfectly idiomatic. That sign has come under attack by the grammar police. One supermarket in Great Britain caved in and changed the sign to “Up to 10 Items.” A small grocery store in my neighborhood has actually changed the sign to “10 Items or Fewer.” But for the most part, supermarkets have wisely ignored the attacks.