I never realized that there’s a huge difference in the usage of the words LESS and FEWER until I read a post that stated otherwise. The usage of the two words is nearly always mixed up because their meanings are very similar.
Fewer is appropriate when writing or speaking about plural nouns; so you have fewer marbles and teeth and rugs. Less is appropriate when writing about units of measurement, singular nouns, or count nouns so great that it makes individual increases meaningless. So you’ll write about less water, less than X kilograms of something, and less money. This last case is a little tricky, as you could easily talk about having fewer bills or coins. The idea is that when you’re comparing vague or uncounted, collective things, it’s usually appropriate to use “less,” while when comparing countable finite items, “fewer” is better. I have used those words incorrectly so many times that it became interesting to see how many times I used them correctly.