1. When you’re trying to say “not as much” of a singular or uncountable noun, you want to use the determiner less. For example:
You should use less sugar in your coffee.
I have less money than my brother.
Could you give me a little less rice?
2. If the noun you’re describing is a plural noun, meaning that it is a countable noun, use fewer. For example:
There are fewer puppies in the pen than last time.
There were fewer tables available for lunch today.
There are fewer people here than last time.