A good question, Giovanna. Fewer for countable nouns. Less for uncountable nouns. It is also a question which fewer and fewer people will be able to answer in the future, as English-speakers are understanding less and less about the basic grammar of their own language...

In general, "fewer" is used for countable nouns and "less" for uncountable nouns, although you will frequently hear native speakers use "less" in both cases. "Less" is also used with adjectives and adverbs. There are fewer students studying calculus this year. We have had less snow this winter than last winter. It's less windy today than yesterday.

We use "fewer" for countable nouns, and "less" for uncountable nouns. Fewer cups, less coffee.