The category of number

English count nouns have singular and plural forms. These nouns are called variable nouns. Singular nouns denotes one, plural ― more than one. Other nouns are used either only in the singular or only in the plural. They are called invariable nouns.
Variable nouns (regular plurals): Noun + -s/-es. The suffix -es is added to nouns ending in:-s, ss, sh, ch, x, z, o: glasses, watches, tomatoes, heroes.

Nouns in -o have the plural in -os: a) after a vowel: zoos, radios; b) in proper names: Romeos, Eskimos; c) in abbreviations: photos, kilos; d) in musical terms of Italian origin: pianos, solos, tangos, sopranos.

Nouns ending in -y, preceded by a consonant, change -y into -ies: stories, flies. Only -s is added: a) after a vowel: boys, toys; b) in proper names: the Kennedys, Marys; c) to compounds: stand-bys, lay-bys.

Note: The plural of abbreviations is sometimes formed in spelling by doubling a letter: Ms (manuscript) ― MSS, p. (page) ― pp., Mr (Mister) ― Messers, MP (Member of Parliament) ― MPs ['em'pi:z] (or MP's), MD (Doctor of Medicine) ― MDs ['em'di:z].
Irregular plurals. They form their plural by:

1) a change of a vowel: man ― men, woman ― women, tooth - teeth, foot ― feet, mouse ― mice, goose ― geese, louse ― lice;
2) voicing (twelve nouns ending in -f (e) form their plural changing -f(e) into-ve: wives, lives, wolves, calves, knives, halves, selves, shelves, thieves, leaves, elves, loaves. In other cases -fs: proofs, beliefs, still-lifes, etc. In a few cases both -fs and -ves forms are possible: scarf ― scarfs (-ves), dwarf ― dwarfs (-ves), handkerchief ― handkerchiefs(-ves);
3) -en plural: ox ― oxen, child ― children, brother― brethren;
4) retaining the singular form in the plural: a) a sheep ― sheep, a swine ― swine, a deer ― deer, a fish ― fish, a craft ― craft, b) nationality nouns in -ese, -ss: Japanese ― the Japanese, a Swiss ― the Swiss; с) quantitative nouns: hundred, thousand, million, dozen, stone (3 dozen eggs, but dozens of people);
5) retaining -s of the singular unchained in the plural: a means ― means, a works  ― works, a barracks ― barracks, a headquarters ― headquarters, a series ― series, a species ― species;
6) plurals of foreign origin: -us — -i [ai]: stimulus – stimuli; -a — -ae [i:]: vertebra ― vertebrae, formula ― formulae; -um — -a [a:]: datum – data; -is [iz] — -es [iz]: basis – bases, crisis ― crises; -on — -a [a:]: phenomenon – phenomena, -ex, -ix — -ices: appendix – appendices.