With the word number, we sometimes use a singular verb and sometimes a plural.

A number of + plural verb

A number of is used with a plural countable noun and a plural verb:

• A number of people have complained about the price increase.

The same is true with a large number:

• A large number of protesters were outside the parliament building yesterday.

The number of + singular verb

• It's a popular school and the number of students has risen again this year.

The numbers of + plural verb

• The numbers of students are rising steadily.