2.208 USAGE NOTE
You can add adjectives to 'a number of' and 'a quantity of' to indicate how large or small an amount or number of things is.
- There are a large number of students.
- We had a limited number of people to choose from.
- The novel provides an enormous quantity of information.
- ...a tiny quantity of acid.
'An amount of' is always used with adjectives, and is usually used with uncount nouns.
- He couldn't stop himself from sipping a small amount of the water.
- He has a large amount of responsibility.
- It only involves a small amount of time.
- There has to be a certain amount of sacrifice.
- They have done a vast amount of hard work.
The plural forms of 'quantity', 'number', and 'amount' are used, especially when referring to separate amounts.
- ...groups which employ large numbers of low-paid workers.
- Enormous amounts of money are spent on advertising.