The difference is very simple: 'Amount' is used for uncountable nouns, and answers the question 'How much?' e.g. 'How much orange juice did you buy?' 'Three litres' = a conversation about the amount of juice bought. 'Number' is used for countable nouns, and answers the question 'How many?' For example: 'How many oranges did you buy?' 'Six' = a conversation about number of individual oranges bought. An amount of water --------> Yes An amount of people --------> No. 'People' is countable. 'How many people?' = a number of people An amount of money -------> Yes An amount of anger --------> Yes, this is possible, but as 'anger' is an unquantifiable abstract noun, it might be more usual to say, for example, 'a level of anger'. 'Quantity' tends to be used for inanimate objects ( i.e. not people), especially in technical, scientific and business contexts. It can be used for both countable and uncountable nouns. I hope that helps.

A quantity of water, lots of water A number of people,a few people, hundreds of people, lots of people An amount of money, a quantity of money, lots of money Anger, very angry, lots of anger, so angry, lots of anger. Anger really is not measurable