The word “less” is often incorrectly used instead of “fewer.”
The rule is that less is used with uncountable quantity and fewer with countable quantity / number, or, more simply put:
The word “less” should be used with singular nouns (less water, less salt) and “fewer” should be used with plural nouns (fewer books, fewer followers).
There is an exception to the rule, though. When an amount of money or a period of time or a distance is used as one unit, it works as a singular word.
For example, I make less than $5000 a year.
Even though dollars can be counted, the $5000 is used as one amount, one total (singular), as opposed to a collection of individual dollars, so the sentence is correct as stated.
Compare these pairs of sentences:
I have fewer books than I used to have.
I have less money than I used to have.
She wrote fewer books last year.
She drank less coffee last year.
He will make fewer chairs next month.
He will make less furniture next month.
This job requires fewer tools than the other job.
This job requires less equipment than the other job.
Thanks for reading.