Well, that's just practical guidance for users, and it's not quite right. It isn't that 'a number of takes a plural verb', it's that the plural verb agreement comes not from the singular NP a number but from the plural NP that follows. The fact that the number of usually follows with a singular agreement is due to the fact that the verb relates to the singular NP the number. I.e., The number of people is growing means that it's the number that is growing, not the people.
I may have to agree with you on that. "
A number of" means "many"; therefore, it is
plural. On the other hand, "
the number of" refers to a mathematical or numerical figure; thus, it is
singular. Some authoritative sources agree with the aforesaid view:
1. The New Penguin English Dictionary [2001] [pg 958] opines that
"the phrase '
a number of'
meaning 'some' or 'several' should be used with a verb in the plural: 'There
are a number of things to discuss'
... When 'number'
means 'the overall quantity in 'the number of' it is used with a singular verb: 'The number of meningitis cases is increasing'
."
2. Oxford Advanced Learner's Dictionary of Current English [1984] [pg 575]:
"The number of books missing from the library is large; A number of books (=some books) are missing from the library."
3. Longman Dictionary of Contemporary English [1978] [pg 749]:
"a number of means some".
4. Collins Cobuild English Usage [2002] [pg 451]:
"A number of things or people means several things or people. You use a plural form of a verb after a number of.... When you talk about the number of people or things of particular kind, you are talking about an actual number. After the number of you use a singular form of a verb."
5. American style guide says that phrases such as
a number of,
a lot of, and
a total of should be followed by plural verbs. The AMA Manual of Style [10th edition] [Sec 7.8.11] says,
"The number is singular and a number of is plural"[3] (thus,
the number of mosquitoes is increasing but
a number of brands of mosquito repellent are available) and the same is true for
the total of and
a total of:
'the total was growing but
a total of 28 volunteers have submitted applications.
6. Chicago style (16th ed) says the same thing at Section 5.9 "Mass noun followed by a prepositional phrase".