amount / number / quantity

Use amount with uncountable nouns – things that cannot be separated or counted.
– There’s a significant amount of traffic in the late afternoon.
– She inherited a large amount of money from her grandfather.
– This recipe requires a small amount of wine.
– I have a huge amount of work to do this week.

Use number with countable nouns – words that can be counted and made plural:
– A number of my friends are teachers.
– A large number of people complained about the restaurant’s terrible service.
– This recipe has a small number of ingredients.
– I have a huge number of tasks to do this week.

The word quantity is a more technical/formal word for number or amount, and it is used for things you can measure (usually objects, not people).
– We have a limited quantity of special-edition hats for sale. (or: a limited number of special-edition hats); hats = countable
– They need a large quantity of cement for the construction. (or: a large amount of cement) ; cement = uncountable