Answer:

A pronoun must agree with its antecedent in number. If the antecedent is singular, the pronoun referring to it must be singular; if the antecedent is plural, the pronoun referring to it must be plural.

Use singular pronouns to refer to the singular indefinite pronouns: each, either, neither, one, everyone, everybody, no one, nobody, anyone, anybody, someone, somebody.
Examples: Each of the students bought their own lunch. (incorrect) Each of the students bought his own lunch. (correct)
Use plural nouns to refer to the plural indefinite pronouns: both, few, several, many.
Example: Both were within their boundaries.
The indefinite pronouns some, any, none, all, most may be referred to by singular or plural pronouns, depending on the sense of the sentence.
Examples: Some of the children have misplaced their toy. (plural) Some of the carpet has lost its nap. (singular)
Pronouns that refer to compound antecedents joined by and are usually plural.
Example: Bill and Joe cook their own meals.
Prounouns that refer to compound antecedents joined by or or nor usually agree with the nearer antecedent.
Examples: Neither Bill nor his friends cook their own meals. Neither Bill nor Joe cooks his own meals.