You want to express the quantity of something but are not sure whether to refer to the amount of it or the number of it.
Let's take these pebbles as an example:
Amount is used for uncountable nouns. This is when there is so much of something we cannot easily count it.
Number is used for countable nouns. This is where there is a limited number of something and we can count it.
In this case there are a lot of pebbles in the picture and we cannot easily count them, so we would say "A large amount of pebbles".
Let's take rice as another example. How would you express this quantity of rice?
We could say there is a small amount of rice on the table. The grains of rice themselves are uncountable in this state.
Or,
We could say there are a small number of rice balls on the table. The balls of rice are countable, we can see there are two of them.
The same also applies when talking about tea:
We talk about an amount of tea when the tea is in a loose leaf form as we cannot easily count the individual leaves.
A number of tea bags when the tea has been pre-packaged into individual bags since we can count these.
