You can use a plural verb with a noun like “number" if it’s followed by a prepositional phrase with a plural noun, but you still need to be careful. (And if you're wondering why "M&M's" has an apostrophe, it's because that is the official product name.)
Every month or two I get a question like this one from Scott. He says he’s been fighting with his editors over a sentence that’s been driving him crazy.
Should it be
We want to know the proportion of all M&M’s that is blue
We want to know the proportion of all M&M’s that are blue.
He says, “I know that ‘proportion’ is singular, so the verb that would go with it is ‘is.’ Clearly though, it’s the M&Ms that “are” blue, not the proportion. But, the proportion needs to be the subject of the sentence. Any other form that we've thought of for this sentence makes it mathematically confusing.”
Ignore Prepositional Phrases for Subject-Verb Agreement
The reason Scott and his editors are frustrated is that there’s a prepositional phrase—“of all M&M’s”—between the subject and the verb. The general rule is that you are supposed to ignore prepositional phrases when they come between a noun and its verb, but in this case, ignoring the prepositional phrase sounds wrong to a lot of people because it contains a—”M&M’s”—and it’s closer to the verb than the noun: “the proportion.”
If you take out the “M&M’s” phrase, you’re left with “We want to know the proportion that is blue,” which sounds fine.
Synesis Says You Can Use the Noun in the Prepositional Phrase
But here’s where it gets really frustrating if you like firm rules: many style guides talk about a concept called “synesis” that lets you use a plural verb in these cases when the meaning of your sentence has a sense of being plural.
“Synesis” comes from a Greek word that means “understanding,” so they’re saying you should go with the understanding of the sentence rather than the strict grammatical structure.
Yes, they’re saying you can ignore the regular grammar rule about subject-verb agreement and ignore the prepositional phrases, especially with nouns of multitude such as “proportion,” “bunch,” “percentage,” and “variety.”
This isn’t a new idea—there are references going back to the nineteenth century, and these are no lightweight, squishy style guides either. Garner’s Modern English Usage, which tends to be on the prescriptive side, says in such sentences you can “justifiably use a plural verb” and says doing so is “grammatically safe.”