Even though less has been used before plural nouns (less words; less men) since the time of King Alfred, many modern usage guides say that only fewer can be used in such contexts. Less, they say, should modify singular mass nouns (less sugar; less money) and singular abstract nouns (less honesty; less love). It should modify plural nouns only when they suggest combination into a unit, group, or aggregation: less than $50 (a sum of money); less than three miles (a unit of distance). With plural nouns specifying individuals or readily distinguishable units, the guides say that fewer is the only proper choice: fewer words; fewer men; no fewer than 31 of the 50 states.
Modern standard English practice does not reflect this distinction. When followed by than, less occurs at least as often as fewer in modifying plural nouns that are not units or groups, and the use of less in this construction is increasing in all varieties of English: less than eight million people; no less than 31 of the 50 states. When not followed by than, fewer is more frequent only in formal written English, and in this construction also the use of less is increasing: This year we have had less crimes, less accidents, and less fires than in any of the last five years.