The word which a pronoun refers to is its antecedent. The pronoun should always agree in number and gender with the antecedent.

Correct:

My friend considers himself an active member of the club.

My friends consider themselves active members of the club.

Incorrect:

My friends consider himself an active member of the club.

My friend considers themselves active members of the club.

Correct:

John bought himself a new tie.

Tina dressed herself for the annual ball.

Incorrect:

John bought herself a new tie.

Tina dressed himself for the annual ball.

If the antecedent is singular, the pronoun will be singular; if the antecedent is plural, the pronoun will also be plural.

Similarly, the gender of the pronoun will be the same as the gender of the noun it relates to.

Remember that while writing or speaking, the agreement between the pronoun and its antecedent should be strictly observed.