According to grammar nazis, there is a rule that “fewer” only modifies countable nouns and that “less” can only modify mass (uncountable) nouns. However, this "rule" is just one of the many superfluous decisions made in the 18th-century by pretentious douchbags who felt that certain things were mistakes simply because they didn't seem elegant or proper enough.
Despite what they say, the rule is not even commonly followed. This is probably because not everyone is anal retentive enough to quibble over arbitrary details that have little to do with how the language is actually understood and used by normal people.
Fewer people showed up to the party than I expected.
I need to eat fewer doughnuts and drink less soda.
This book has fewer than 200 pages.
I have one fewer card than you.
We have fewer than 15 minutes to get there!
I have fewer than 20 dollars.
It’s fewer than five miles away.
She’s fewer than six years old.