The phrase "the number" requires a singular verb. The phrase "a number" requires a plural verb.

The number of frogs in the pond is twice the number of fish.

A number of protestors are gathering outside the capitol building today.

When you see either phrase – "the number," or "a number" - disregard the singularity or plurality of the noun following it. If you're having a hard time remembering to do so, try crossing out all information between the "number" phrase and the first verb.

The noun does not impact the verb in any way whatsoever, because "the number" and "a number" are the subjects of the sentence, not the nouns following them.

G. Subject / Verb / Object

I. Sample Questions