Two of the most common mistakes I see in writing are confusing less and fewer, and confusing number and amount.
They both relate to things that are countable versus not.
If you can count it, like 1,2,3…, you would use fewer and also use number.
- 10 items or fewer
- A large number of frogs
- Fewer people
- Fewer dollars
If you can’t, you would use less or amount.
- Less sugar
- A smaller amount of rain
- Less population
- A larger amount of money
Please don’t say you have an amount of people. That could be messy.
If you like your grammar lessons with words like “prescribed comparative,” you might like Wikipedia’s take on less/fewer.