There are a number of words, called collective nouns, one of which is number and which can work with other words to create singular or plural subjects. Whether the constructions are considered singular or plural subjects depends on what is going on in the sentences in which they are found.

If the entire collection as a whole is the subject of the sentence, or the focus of what is being discussed, then the construction is considered a singular subject. If the individual members of the collection are the subject or focus, then the construction is considered a plural subject.

Meanwhile, regarding some of these I have noticed some disagreement, particularly regarding the word group, which according to some is always singular. That does not make sense to me. A number, the majority, and the family can all be plural, but a group is always singular?? Why? I say that group can be singular or plural too.

Meanwhile, the GMAT has gotten away from using arcane concepts as decision points in its sentence correction questions. So I doubt you will see a question that requires your deciding which answer choice is correct by looking at whether group is singular or plural. I surely hope not. Also, there are generally multiple decision points in each answer choice. So if you are not sure about one, probably you can find another.

Anyway, here are some examples.

A number of people have come to us with ideas for a new plan. (This emphasizes that individuals have come with ideas. So the subject is plural. A number is basically like some. Some people have come to us.)

The number of students is greater than the number of teachers. (This is discussing not the individual students but rather the number of students.)

The majority of the senators say that they will reject the new bill. (This is about the senators.)

A majority of the votes is required to pass the bill. (This is about a majority.)

A couple of people were standing there. (This is about the people standing there.)

Generally one couple at a time fits in the wedding chapel. (This is about the couple.)