Now this is getting nit-picky. There are generally two schools of thought when it comes to the less vs fewer rule.
Prescriptive grammarists - those who focus on correct or incorrect use of a language - use fewer when a noun is a countable object or concept. In contrast,less should be used with nouns that can’t be counted, or when the item/concept is combined into a group, or doesn’t have a plural.
E.g. “I had less than $50 in my wallet” vs “I had fewer than three $20 bills in my wallet.” The difference here is quantity vs individual items.
However, descriptive grammarists - those who focus on how the language is actually used day-to-day - would agree that this is not the rule that would describe its most common usage today.
Interestingly, the origins of this rule came about as an incorrect generalisation of a personal preference of the grammarist Robert Baker in 1770, taken from his book Reflections on the English Language.