Fewer than and less than are commonly used interchangeably, but these words have some differences. The main difference between the two is the quantity that is addressed.
Fewer than is used to refer to things that can be counted (countable things). For example, fewer schools and fewer cars.
On the other hand, less than is used to refers to things that cannot be counted (uncountable/mass things). For example: “The rate of drug use is less than it was last year.”
Also, less than can be used when referring to plural nouns for amount, time, and distance. For example: “I have less than 10 dollars left”.