Words that refer to a group of people or objects but are singular in form are called collective nouns.
Examples: army, audience, choir, committee, team, mob, number
Verbs and pronouns used with collective nouns are either singular or plural, depending upon the meaning of the group word. If you consider the noun a unit, then use a singular verb and relative pronoun:
The team wants to win its division.
The committee is made up of department heads.
The noun in both sentences is considered a single unit, so the verb in each case is singular, and the pronoun in the first sentence is singular as well.
If you consider the collective noun a group of individuals, use a plural verb and plural pronouns:
The committee are arguing among themselves.
Both the verb and pronoun are plural, because the committee is viewed as a group of individuals, not a unit.
The collective noun “number” may be singular or plural. When it is considered arithmetical (you can count it, 1-2-3…), it is usually singular:
The number of students assigned to the new high school is higher than expected.
This number of students can be counted.
A number of students have already chosen their classes for the new semester.
This “number” is a concept, typically referring to a relatively large group.
If the noun following “number” is also collective, follow the guidelines above:
The number of people signed up for the reunion is growing daily.
A number of people are in disagreement with the proposed regulations.
I’ve compiled a few online lists of collective nouns for your pleasure and edification: