Amount is how much there is of an uncountable mass (the amount of water); number is how many there are of separate, countable things (the number of bottles). Use amount with mass nouns and 'less', number with plural countable nouns and 'fewer'.
amount + uncountable mass (less water); number + countable plural (fewer bottles).
These two both seem to answer 'how much stuff?', which is why they get swapped — but they sort the world differently. Amount measures a single uncountable mass: water, sand, time, money. Number counts separate units you could tally one by one: people, errors, coins. The quick test: if you can make it plural and count it, it takes number; if it only comes as a mass, it takes amount. The very same split decides less vs fewer.
amount
An amount is how much there is of something you measure rather than count — a mass or quantity treated as a single whole: the amount of water, sand, time, or money. Because the thing is uncountable, you weigh or gauge it instead of tallying separate units, and it pairs with 'much' and 'less' (a large amount, a small amount). It comes from Old French amonter, 'to mount up'. Set it against number, which is for separate, countable things. As a verb, to amount to is to add up to a total or be equivalent to.
number
A number is how many there are of separate things you can count one by one — people, errors, marbles, days. Each unit is distinct, so you tally them rather than weigh or measure them, and the result is exact: three, forty, a thousand. It goes with 'many' and 'fewer' (a large number, a small number). From Latin numerus. Set it against amount, which is for a single uncountable mass. To say 'a number of' is a quiet way to say 'several' without fixing exactly how many.
| amount | number | |
|---|---|---|
| Meaning | how much of an uncountable mass | how many separate countable things |
| Use with | mass / uncountable nouns | plural countable nouns |
| Pairs with | much, less, a little | many, fewer, several |
| Operation | measured (weighed or gauged) | counted (tallied one by one) |
| Example | the amount of water | the number of bottles |
| Common trap | amount of people ✗ | number of people ✓ |
Ask whether you could count the thing one by one. Water, sand, time, money, work — you cannot tally them, so they take amount (and much/less): 'a large amount of work'. Bottles, people, errors, days — you can count them, so they take number (and many/fewer): 'a large number of errors'. Picture the sugar both ways: pour it loose and a smooth mound just grows, asking how much → amount; drop it as cubes that land one by one, asking how many → number. The same rule sorts less (mass) from fewer (count).
amount
- iA vast amount of data is generated every second.
- iiWe wasted a fair amount of time looking for parking.
- iiiThe amount of sugar in the recipe can be reduced by half.
number
- iA record number of applicants sat the exam this year.
- iiThe number of errors dropped sharply after the redesign.
- iiiOnly a small number of seats remain.
If the noun is uncountable (water, advice, information), use amount; if it is a plural you can count (bottles, people, ideas), use number. The same logic separates less (amount) from fewer (number): 'less water', 'fewer bottles'. Money is uncountable (the amount of money), but coins are countable (the number of coins).
This is a grammar-accuracy point examiners actively look for, especially in IELTS Writing Task 1. Use 'amount of' with an uncountable noun (a large amount of information) and 'number of' with a countable plural (a number of factors); the same split governs 'less' versus 'fewer'. 'A number of' also takes a plural verb ('a number of studies show'). Handling countability correctly — 'a small number of respondents', 'a large amount of rainfall' — is a frequent band-score differentiator.