Supermarket express lanes have been quietly teaching this wrong for fifty years. "Ten items or less" is on thousands of signs, and it is the single most cited example of an error in English. The complication is that the people doing the citing are on shakier ground than they think.
Used with plural count nouns — things you could number individually.
Used with mass nouns — quantities measured rather than counted. Also with time, money and distance, even though those are countable.
Quick way to remember
Ask whether you would count the thing or measure it. Count it, use "fewer." Measure it, use "less." Then remember the three exceptions — time, money, distance — which take "less" regardless: less than an hour, less than fifty dollars, less than a mile.