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Amount/quantity/number

Use amount of before singular things you cannot measure (единственное число, неисчисляемые).

Use quantity of before singular or plural things you can measure (единственное и множественное число, исчисляемые).

Use number of before plural things you can measure (множественное число, исчисляемые).

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)

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.

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.)

Read more at http://www.grammar-monster.com/easily_confused/amo...number.htm#3o4gEel97OW3uzEA.99