Number
From 'Grammar' part of The ABC Of Plain Words by Sir E Gowers (1951)

(i) Like other collective nouns, number may take either a singular or a plural verb. Unlike most of them, it admits of a simple and logical rule. When all that it is doing is forming part of a composite plural subject, it should have a plural verb, as in:

A large number of people are coming today.

But when it is standing on its own legs as the subject it should have a singular verb, as in:

The number of people coming today is large.

The following are accordingly unidiomatic:

There is a number of applications, some of which were made before yours.
There is a large number of outstanding orders.

The true subjects are not "a number" and "a large number" but "a-number-of-applications" and "a-large-number-of-outstanding-orders". Is should therefore be are in each.

Of the following examples the first has a singular verb that should be plural and the second a plural verb that should be singular.

There was also a number of conferences calling themselves peace conferences which had no real interest in peace.
The number of casualties in H.M.S. Amethyst are thought to be about fifteen.

(ii) There is no point in writing a number of when what you mean is some, several, or many; you merely use three words where one is enough.