You may have heard the traditional "countable" rule about less versus fewer, but there's also a better rule you may not have heard that covers some of the exceptions to the traditional rule.
One Less Complaint
Using the “singular or plural” rule also explains another “exception.” People often think phrases such as one less banana are wrong because you can count bananas, but one less banana is correct because it is singular and you use less with singular nouns.
One less banana and similar phrases put you in a tricky situation because they are correct, but many people think they are wrong. For example, I got grammar-related complaints after Gardasil launched its “one less person affected with HPV” ads because many people thought it was grammatically incorrect. Therefore, I recommend avoiding the construction whenever possible. It’s better to rewrite your sentence than to have people think you’ve made a mistake or to knowingly use the wrong word by writing one fewer X. You really can’t win whether you write one less banana or one fewer banana. So rewrite. Instead of telling your caterer We need one less banana in the fruit bowl, avoid the controversial sentence by asking her to Take one banana out of the fruit bowl.