NGS Style Manual‎ > ‎- F -‎ > ‎

few, fewer, less, lesser

Use few to mean an indefinite small number whose size depends upon the context:
            for example, among U.S. citizens a few may number in the thousands

Use fewer only with a plural noun for things that can be counted one by one:
            fewer than ten men
            fewer chairs

Do not write fewer in number.

Use less with a noun that does not have separately countable parts:
            less water, less activity, less money.

Also use less with a plural noun that is a mass, a measurement, or a total, especially in expressions following less than:
            less than two hours
            less than $30
            less than five miles
            less than three gallons
            a total population of less than five million
            less than 10 percent of them are now nomadic
            less than half of all Roma children regularly attend school

Do not hyphenate less or lesser in compound modifiers:
            a less useful book
            the lesser known of the two
          
See also couple, few, several.