"Less" and "fewer" mean different things. "Less " refers to a smaller quantity of anything that can be measured by volume. "Fewer" refers to a smaller number of anything that can be counted.
So you might have less water in a reservoir, less gas in your tank, less spring in your step or less hope for future generations.
You might have fewer students in this year's English class, fewer mosquito bites on your arm, fewer chances to ski since Global Warming began to make itself manifest or fewer grammatical mistakes in this essay than the last one.
However, if you use the phrase "less people", that conjures up the idea that you have ground up the people in question, possibly in a gigantic blender, and poured some out. A truly horrific notion, as I'm sure you'll agree.