Less.

Should not be misused for fewer.

Poor: He had less men than in the previous campaign.
Better: He had fewer men than in the previous campaign.

Less refers to quantity, fewer to number. “His troubles are less than mine” means “His troubles are not so great as mine.” “His troubles are fewer than mine” means “His troubles are not so numerous as mine.” It is, however, correct to say, “The signers of the petition were less than a hundred, “where the round number, a hundred, is something like a collective noun, and less is thought of as meaning a less quantity or amount.

From: William Strunk, Jr., The Elements of Style, 1918.