When a sentence begins with “A number of,” should the verb that follows be singular, or plural? For example, when a sentence refers to a number of objections being raised, is was correct, or should you use were? In this case, number stands in as a vague reference to the quantity of objections, but the objections themselves are the focus of the sentence: “A number of objections were raised.”
(This last sentence is passive; it might be better to construct the sentence more actively by identifying — and emphasizing — those who objected: “Community members raised a number of objections during the public-comments period.” Occasionally, though, the identity of the actor or actors is irrelevant, or the writer wishes to deemphasize or disguise their identity. That’s why passive construction isn’t categorically wrong — it’s merely less direct.)
However, when the sentence begins with “The number of,” the verb that follows should be singular: “The number of chairs available is 500,” for example, is correct, because the subject of the sentence is number, not chairs, and number is a singular noun. (Of course, “Five hundred chairs are available” — avoid starting a sentence with a numeral — is more direct and concise, but, again, sometimes a more relaxed syntax is desirable.)
The difference may seem negligible, but consider that in an “a number of” construction, though the plurality of phenomena in the sentence is important, the identity of the phenomena — objections, in the first example above — is the essential information. Likewise, in the second example, the reader needs to know what objects the stated quantity refers to, but the point of the sentence is the quantity; hence, number is the key word.
The same principle applies when the sentence begins with the expletive there or here, followed by a verb: “There are a number of objections” and “Here is the number of chairs you requested.” Again, these sentences are not necessarily ideally constructed, but when you need to write in such a syntactical style, remember, “A number . . . are” and “The number . . . is.”