Fewer or Less

So the rule according to Oxford Dictionaries is this:

Use fewer if you’re referring to people or things in the plural (e.g. houses, newspapers, dogs, students, children). For example:

Fewer people are going to theaters these days due to stagnation in Hollywood and the rise of online services prepared to take greater risks creatively and has nothing whatsoever to do with Rotten Tomatoes.

Use less when you’re referring to something that can’t be counted or doesn’t have a plural (e.g. money, air, time, music, rain). For example: 

If you spent less time trying to repeat your past successes and more time coming up with new ideas maybe things would be different.

Remember; not as much = less, not as many = fewer.

Of course, the argument can always be made that language is determined by usage and fewer and less are used interchangeably so often, and have been for many decades, that the smart thing to do is really just to avoid needless headaches and accept it. In which case sorry, Stannis Baratheon, but you’re wrong. Also we were told what a great military leader you were but never saw any evidence of it, so screw you anyway.