Quite often even native speakers make the mistake of using "less" instead of "fewer".
Should you be in a situation where you may not want to infuriate Stannis Baratheon, the question then poses itself - so, how do I use these words without making a fool of myself?
a) Use "fewer" when you are referring to objects or persons in plural - e.g. "I saw fewer people today than I did yesterday". "There are fewer youtube videos uploaded this year than in 2016." "Fewer women choose to study engineering each year." "Fewer than thirty boys developed a disease."
b) Use "less" when referring to something which cannot be counted directly - e.g. "I earned less money this year." "Let's spend less time together." "The palace is less than two miles away."
Voilà!