Countable nouns are those where we could count a discreet number of units. Uncountable nouns are those where we cannot do this. For example, ‘water’ is an uncountable noun, whereas ‘litres’ is a countable noun.
We use ‘amount’ when describing uncountable nouns and ‘number’ when referring to countable nouns.
Thus:
He spilt a large amount of flour.
She lost a large number of coins
A large amount of beer was quaffed.
A large number of bottles were emptied.
The amount of data is mind-boggling.
A number of new inventions were described.