出典:Wiktionary
Some[*] regard the use of the determiner less with countable quantities to be incorrect, stating that less should indicate only a reduction in uncountable quantity, or in size or significance, leaving fewer to indicate a smaller numerical quantity. For example, they suggest saying less sugar, but fewer people, not less people. Such a rule can allow distinctions such as:
Nevertheless, less has been widely understood and commonly used as a synonym for fewer since it first appeared in Old English as læs.