In English grammar, "number" is just a term used to indicate singular vs. plural, typically for the purposes of subject-verb agreement. It doesn't have anything to do with numbers like five or twenty. If someone asks, "Do the subject and verb agree in number?" then they are asking if a singular noun has a verb conjugated in the singular, or if a plural noun has a verb conjugated in the plural.
Don't be confused by the fact that most plural nouns end with "-s" and most verbs conjugated for the singular end with "-s." This is just a coincidence -- the "-s" conjugation on a verb has nothing do with the "-s" that makes most nouns plural. The verb "walks" is not a "plural" verb because verbs can't be singular or plural (that designation is for nouns/pronouns). The verb "walks" is conjugated for a third-person singular subject, as in "He walks to school every day." When it is conjugated for a plural subject, the verb is "walk," as in "They walk to school every day."