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.