“Number: A grammatical category used in describing parts of speech that show contrasts of plural, singular, dual, etc. In English, the number system is basically a two-term contrast of singular and plural, shown in nouns and some pronouns and determiners, and to some extend in verbs. Even dual words, such as both, either, neither, take singular or plural verb concord: both taking the plural; either, neither usually taking the singular. English nouns, as far as number is concerned, can be divided into: singular only, plural only, and words that can be both. Singular-only nouns are: (1) Uncountable nouns which can occur with such uncountable-specific words as much, little: much money, little sugar. (2) Most proper nouns: Edinburgh, the Thames (in which other restrictions apply). Plural only nouns are (1) Countable: people in six people, but not in the European peoples. (2) Usually uncountable: not enough clothes (not six clothes); many thanks (not five thanks); trousers (a pair of trousers but not usually three trousers). The vast majority of countable nouns can be both singular and plural (book/books, fox/foxes, mouse/mice), but a few have no distinct plural form (as with one sheep/three sheep). Many nouns, however, have both countable and uncountable uses, in which case they may have a plural in some uses (What an excellent wine/What excellent wines!) but not in others (I never drink wine). Pronouns having distinct singular and plural forms include personal, reflexive, and possessive. Number contrast is neutralized with you, but the second-person reflexive forms distinguish yourself and yourselves. Demonstrative pronouns also have separate forms, singular this, that being used with singular countable nouns (this restaurant) and with uncountable nouns (this food). Number contrast in verbs, except in the verb be, is confined to the distinct third-person singular tense form (look/looks).”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.