Deciding whether to use less or fewer can be difficult.
The confusion happens because you often see mistakes in public signs, for example like this one here:
The sign is wrong!
'Less' is used with things that you don't usually count, so this sentence is correct:
A small car uses less fuel than a big car.
Generally speaking, we don't say 'a fuel' or 'two fuels'. Fuel is an uncountable noun and uses 'less'.
'Fewer' is used with things you can count, so this is sentence is correct:
Fewer people came to your party than to my party. I must be more popular than you!
You can count the number of people who came to the parties, so you use 'fewer'.
So the the sign in the shop should say: 'Ten items or fewer'!
The word 'people' doesn't have 's' on the end and this might be one reason why sometimes people say 'Less people' instead of 'fewer people'. But you still count people (it is just an irregular plural without an 's') so you should use 'fewer'.
But I agree it's not always that simple. Should we say: 'fewer clothes' or 'less clothes'? If you're chatting in an informal situation probably no one will notice or care which one you use! If you want to be accurate, 'fewer clothes' would be grammatically correct. 'Clothes' is a plural word and plural words use 'fewer' even though the word doesn't have a singular form. 'Fewer clothes' or 'less clothing': it's the same thing!
You can read read more about these structures from the Cambridge Dictionary here.
Other articles about grammar on English for University.Com are here.