The antonym of less, which is more, applies to either number or quantity. When qualifying a number, it ought not to be contradicted by an associated word that applies only to quantity--e.g., much or little. With little more than 500 water skiers in a town population of 8,000 requires few or a noncommittal hardly in place of little. The set phrase more or less is an encouragement to the confusion of quantity with number: He had chosen one of those ruthless competitors, more or less of whom are to be found on every team / The tattoo maniacs and chain-swinging bikers, of whom more or less were always hanging about the dock. In these, more or less might well be replaced by some, which, like more, serves for both quantity and number.

An occasional construction in which the ideas of quantity and number are hardly distinguishable will be equally tolerant of less and fewer: The three million seasonal farm workers normally work less than 150 days in a year. Here 150 days can be felt as either a specified number of days or a unitary measure of time (as five months or less than half a year would be). Sometimes an ostensibly numerical expression is unmistakably a unitary measure and, as such, excludes fewer. We take a billion dollars as a sum of money, not as a number of units; fifty feet as a measure of distance, not as one foot added to forty-nine other feet; thirty minutes as a stretch of time, exactly like half an hour. With these expressions a singular verb is appropriate (A billion dollars is more easily printed by the government than it used to be / Fifty feet is too short a distance), and the quantitative less is therefore correct in comparisons; fewer would sound absurd.

Source: Modern American Usage, Jacques Barzun, 1966.