“Number: A feature of nouns, pronouns and a few verbs, referring to a singular or plural. A subject and its corresponding verb must be consistent in number; a pronoun should agree in number with its antecedent. A solo flute plays; two oboes join in.”
Excerpted from: Strunk, William Jr., and E.B. White. The Elements of Style, Fourth Edition. New York: Longman, 2000.