Fewer is used with quantities
The rocket ship had fewer than 13 monkeys on it)
There are fewer than 1.5 conscientious calculators in the room. (Hey, there’s that “than!”)
Do not say: the rocket ship had less than 13 monkeys.
Let’s move onto less.
Less is used as an ambiguous comparison.
Ralph was less handsome than Hanson.
I was less than inclined to engage with Hanson because he was an unscrupulous fellow.