Pronouns, like nouns, have singular and plural forms. Plural pronouns take the place of plural nouns. Singular pronouns take the place of singular nouns.

The mob of customers shouted their objections.

-- The plural pronoun "their" refers to the singular collective noun “mob."

The mob of customers shouted its objections.
The customers shouted their objections.

Pronoun agreement is especially confusing when indefinite pronouns such as "everyone," "everybody," "anyone," and "anybody" are used.
Treat "everyone" and "everybody" as plural pronouns:

Does everyone have their tickets ready?
I told everybody to stay in their seats.

Singular indefinite pronouns, such as "anyone," "anybody," "a person," "someone," and "somebody" present problems because there are no singular gender-indefinite personal pronouns in English.