Fewer or less: which is correct?

Use fewer with countable things and less with uncountable quantities. Fewer people, fewer emails, fewer mistakes. Less traffic, less patience, less flour. The supermarket sign reading ten items or less is the most cited example of the error, and technically it should read fewer, which is why some chains quietly changed their signage after decades of complaints.

The exceptions are where confident people get it wrong in the other direction. With time, money, distance, and weight, standard usage takes less even though the units are countable, because the phrase refers to a total amount rather than individual items: less than five miles, less than twenty dollars, less than three hours. Saying fewer than twenty dollars is technically defensible and sounds wrong to nearly every native speaker. Also note that less has been used with countable nouns for over a thousand years in English, so the strict rule is a relatively modern prescription rather than an ancient law. The practical guidance stands anyway: use fewer for countable items in careful writing, use less for amounts and for time, money, and distance, and do not correct strangers about it.