Number is that quality of a word which shows whether it refers to one or to more than one.
Singular Number refers to one. Plural Number refers to more than one.
Plurals of singular nouns are formed according to the following rules:
1. Most nouns add s to the singular; as, boy, boys; clock, clocks.
2. Nouns ending in s, ch, sh, or x, add es to the singular; as, fox, foxes; wish, wishes; glass, glasses; coach, coaches.
3. Nouns ending in y preceded by a vowel (a, e, i, o, u) add s; as, valley, valleys.
When y is preceded by a consonant (any letter other than a vowel), yis changed to i and es is added; as, army, armies; pony, ponies; sty, sties.
4. Most nouns ending in f or fe add s, as, scarf, scarfs; safe, safes. Few change f or fe to v and add es; as, wife, wives; self, selves.
The others are: calf, elf, half, leaf, loaf, sheaf, shelf, staff, thief, wharf, wolf, life. (Wharf has also a plural, wharfs.)
5. Most nouns ending in o add s; as, cameo, cameos. A number of nouns ending in o preceded by a consonant add es; as, volcano, volcanoes.
The most important of the latter class are: buffalo, cargo, calico, echo, embargo, flamingo, hero, motto, mulatto, potato, tomato, tornado, torpedo, veto.
6. The following common words always form their plurals in an irregular way; as, man, men; ox, oxen; goose, geese; woman, women; foot, feet; mouse, mice; child, children; tooth, teeth; louse, lice.