You need to understand the difference between amount and number.

There are things we can count and things we cannot count. With countable nouns we use number of and with uncountable nouns amount of. We can talk about the amount of time we work or about the number of hours we work.

A large number of cars had stopped behind the lorry.
The number of spectators was impressive.
She only drank a small amount of water.
I hadn’t expected that amount of work.

We can also use plural forms:
He drank vast amounts of beer.
Here you will find statistics related to numbers of taxpayers and registered traders.

With amount we say how much of something is present.
With number we talk about how many there are.

Notice how the verb form changes:

The number of students has increased every year since 2015.
The verb is in the singular because the main subject here is number. The word students could be replaced by another word such as cars, newspapers, attacks, etc.

A number of students have published a campus journal.
The verb is in the plural because the main subject is students. A number of can be replaced by, for example, some.

To sum up:
The number of … has the singular form of the verb.
A number of … has the plural form of the verb.

Both amount and number can also be used as verbs:
How much did it amount to?
Number the parts from 1–10 according to how you rate their functionality.

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