This decision can cause some problems. Luckily, it is easily solved. The most common mistake is to use 'less' when 'fewer' would be correct. It is unlikely you would make the reverse error.
Less is used with things/material that cannot be counted or separated into individual parts. You can not count orange juice, sunshine, sand etc (although you can count litres of orange juice, hours of sunshine, grains of sand etc)
I would like less custard please. (Custard is not countable)
You get less jam in a smaller jar. (Jam is not countable)
There is a lot less rain in the Greek islands than in Bristol. (Rain is not countable)
Once I heard what happened, I had far less sympathy with her. (Sympathy is not countable)
Fewer is used with discrete things that can be separated or counted. CDs, sausages, cows, people etc can be counted. By far the most common mistake is to use 'less' when 'fewer' is needed.
If only there were fewer people in this bus. (People are individual, countable things)
He decided he should buy fewer CDs. (CDs are individual, countable things)
Steve always buys fewer drinks than the rest of us! (Drinks are individual, countable things)
I will have to take fewer holidays this year. (Holidays are individual, countable things)
Test your understanding of less/fewer in this exercise.