We need to add an additional ending to the words in order to express the idea of plural. It is passed on to those other words that come in contact with the noun in question. Verbs are the words that express action or development. Verbs change, then, to indicate if one or several people take place in the action. Finally, another group of words that has the property of number is the pronouns.
Pronouns are words that replace nouns.
In Spanish too pronouns have the number property, which they inherit or acquire from the noun they represent. Estudio con la hermana de Juan. Estudio con las hermanas de Juan.