A version of this post appeared on The Week:
The set of words "there's a number of reasons" yields almost 3 million hits on Google. But the phrase "there are a number of reasons" returns more than 63 million results. And perhaps the least natural-sounding possibility, "there is a number of reasons," gets about 73 million.
So there's clearly a division of opinion, but why? Most likely because there are two considerations at issue here and they're sometimes at cross-purposes:
(1) Is it acceptable to use there's when the noun following it is plural? and (2) Is a number of reasons singular or plural?
The first issue is the simpler one. English has a long history of using there as a filler word that allows us to declare the existence of something while putting the thing itself in the more emphasized position after the verb. When there introduces a sentence in this way, the verb, as a general rule, agrees with the noun following it. There is an egg on the table. There appear to be two eggs on the table. There are two eggs on the table.
So where does that leave there's? Well, there's is commonly treated in casual and dialectal speech as a fixed, unchanging item. Many well-educated people, for instance, would say "There's two eggs on the table" without thinking, but they wouldn't dare write it that way. So when it comes to there's, it all depends on just how comfortable you are with informality.
The more interesting—and more hotly debated—question is whether a number of reasons is singular or plural.
Those in the "singular" camp point out that a number is the head of the noun phrase and is singular. Those in the "plural" camp will point out that a lot, a bunch, a hundred, a thousand, and a million are all technically singular in form, too. Terms such as these—including also a dozen, a pair, and a couple—act as singular nouns if you're treating the dozen, pair, or couple as a unitary set, but drift grammatically into pluralness otherwise. "A bunch of people is coming," if you think about it, suggests a slightly different image than "a bunch of people are coming."
The same issue holds for a few other similar terms, including a large percentage and the majority. If we look at historical usage data, we find that "a large number are," "a large percentage are," and "the majority are" all beat out their "is" counterparts, sometimes by large margins, at least since 1800. Could all those users be wrong? Or is it perhaps more likely that those in the "singular, you idiot!!!" camp are misanalyzing the grammar?
Here's one way to test it: Use other verbs instead of is/are. Take this sentence: "Among students, a large percentage graduates from high school, the majority finishes college, and a fair number goes on to graduate school." Does it sound more natural with graduate, finish, and go on? Try this: "We have many guests, and a number of them uses the shower every day." Better with use?
Here's another way to test it: Look for distinctions in meaning. We know we can say, "The majority is always right," meaning that, as a body, those constituting the majority prevail. But what if we say, "The majority are always right?" That could be construed to mean that of a particular population, the set of those who are always right is greater than the set of those who are not. So we do have a distinctive plural usage of the majority.
Similarly, "A high percentage get an A on the test" likely means that very many get an A. On the other hand, "A high percentage gets an A on the test" means that a high percentage (say, 90 percent) on the test will get you an A.
So, if I say, "There is a number of reasons written on the paper," expect to see a figure such as 5, 12, or 67. If I say, "There are a number of reasons written on the paper," expect to see the reasons themselves. And if I say, "There's a number of reasons written on the paper," well, it really depends on just how casual I'm being with my speech—but don't expect me to use it in earnest in formal written English.
There are a number of reasons I wouldn't.