Inquiring minds are so inquisitive! One of the customers now asks whether one should say “a number of [insert items here] are” or “a number of [insert items here] is”….
Honestly, the things some people worry about!
The choice of a singular or plural verb with nouns that describe amount or quantity—such as “number” or “percentage”—is governed by a principle called "synesis," a concept developed in the 1860s by the English grammarian Henry Sweet (undoubtedly known to his intimates as “Sweetie”). "Synesis," from the Greek for "sense" or "understanding," is a grammatical construction made according to the meaning to be expressed, rather than according to strict syntax.
The classic example of synesis is "A number of items are...." The same principle applies to units of measure that have a collective sense: "Two quarts of Southern Comfort is all I can drink at one time." Some grammarians take the principle of synesis so far that they will accept the construction "If anyone calls, tell them I'm out," the meaning being tell all the people who call that I am out (out cold, that is, from all that Southern Comfort).
It is customary in the case of "a number of items" to use the plural verb; but in the case of "the number of items" the singular is more usual: "A number of people were invited to my party this year," but (since I served only Southern Comfort last year) "the number of people who accepted was small."
A number of people have requested that the Bard of Buffalo Bayou cease and desist from his quasi-poetic activity; in fact the number of such people is astronomical. But their pleas (usually without “please”) have fallen on deaf ears.
Whenever “number” rhymes with “lumber,”
It means it’s artithmetical.
But when “number” rhymes with “dumber,”
It’s something anesthetical.