For quantifying discrete objects, you should use "fewer", not "less". "Less" is used for aggregates, continuous media, and numeric values. So, "fewer people", but "less water".
You needn't do this. Some random bloke thought it'd be a good idea years ago, wrote it down in a book. A certain type of pedant says to themselves, oh somebody wrote it down in a book - that's the rule them. Nope.
You can't use fewer in some places, but the insistence that if you can use it then you must is an zombie rule. If "less" sounds right it's fine.