Use fewer, not less, with individual items, which can usually be counted: fewer than seven samurai (not less than seven samurai). Less is used for masses: The pessimist retorted that a glass half empty has no less
water than a glass half full.
Not all numbers need fewer. Write less than £200, less than 700 tonnes of oil, less than a third of Americans, because these are measured quantities or proportions, not individual items. Time, distance and other things measured on a continuum also take less; in less than six weeks, less than six feet tall.
One less before a noun (boron has one less electron than carbon) is acceptable. Standing alone, one fewer is ne: He had not 100 problems, but one fewer. One fewer also precedes than: the party won 16 seats, one fewer than at the last election.