Q. I have a question that my colleague and I can’t find a definitive answer to, and that is whether less or fewer is used with countable, but singular, nouns. For example, “one less/fewer group,” “one less/fewer number,” and so on.
A. If the countable noun is plural, choose fewer; if it’s singular, choose less. (When CMOS says to reserve fewer for countable things, it’s talking about plural countable things. When it says to reserve less for mass nouns, it means singular mass nouns.) One is always singular: there is one less food group in the new pyramid; there is one less number in this column. Two (or more) is plural: there are two fewer food groups in the new pyramid; there are three fewer numbers in this column.