'amount', 'quantity' and 'number' : the difference

Use 'amount of' before singular things you cannot measure.
Use 'quantity of' before singular or plural things you can measure.
Use 'number of' before plural things you can measure.

There are subtle differences between 'amount of', 'quantity of' and 'number of'.

+ 'Amount Of':
The term 'amount of' is used for things you cannot measure. It usually precedes a singular word.

Examples:
"I undertook an inordinate amount of work."
(work - singular and cannot be measured or counted)

"She had a certain amount of respect for the sales team, but she always dissented when they spoke at meetings."
(respect - singular and cannot be measured or counted)

"It is not difficult to see where that amount of hate derives."
(hate - singular and cannot be measured or counted)

WRONG: "The amount of women in London who flirt with their own husbands is perfectly scandalous. It looks so bad. It is simply washing one's clean linen in public." (Oscar Wilde)
(women - plural and can be counted. 'The number of women...' )


+ 'Quantity Of':
The term 'quantity of' is used for things that you can measure. It is usually applied to inanimate objects. Some grammar references may advocate that it can only precede a singular word; however, this view is now considered outdated.

Examples:
"I took control of a large quantity of money."
(money - singular and can be measured or counted)

"The ship was only carrying a large quantity of mangoes."
(mangoes - plural and can be measured or counted)
('Number of' could also be used in this example. 'Number of' is preferable as it is unlikely to be considered incorrect by some readers.)


+ 'Number Of':
The term 'number of' precedes a plural, countable noun. It can be applied to both animate and inanimate objects.

Examples:
"The disease affected a large number of camels in the town."
(camels - plural, animate and can be counted)

"The ship was only carrying a large number of mangoes."
(mangoes - plural, inanimate and can be counted)
('Quantity of' could also be used in this example.)


+ Hot Tip: 'LESS' AND 'FEWER'
'Amount of' should be used with a singular word, and 'number of'\ should be used with a plural word. Similarly, 'less' should be used before a singular word, and 'fewer' should be used with a plural word.

+ Beware: 'QUANTITY' OR 'AMOUNT'
Some people may have different ideas on what is measurable.

For example:
"Do I need that amount of sleep?"

Is sleep measurable? If you judge that sleep can be measured in hours, then 'quantity of sleep' could be an acceptable alternative. In general, tangible objects (i.e., things you can touch) attract 'quantity of' and intangible objects attract 'amount of'.


+ Take the Test:
Select the correct version:

- "Two reams are insufficient. I'll need a larger amount / quantity of paper tomorrow."

- "The agency is considering increasing the amount / quantity of meat per person."

- "Do I need that quantity / amount of sleep?"

- "Did the advert attract a greater amount / number of applicants?"

http://www.grammar-monster.com/easily_confused/amount_quantity_and_number.htm