What makes possessive noun singular or plural?

Answer 1

Only a plural possessive can be identified solely by its ending: a final apostrophe means it is plural.

There are two endings that are the sign the possessive case in English nouns. One is for all singulars and one or two plurals, the other is for plurals only.

All English singular possessives are formed by adding 's: the girl's hat, the boss's office, Socrates's wife. (Some plural possessives are also formed this way, if they do not normally end in s: the children's room, women's shoes, the bacteria's mutation.)

All English plural nouns ending in s form the possessive by adding the apostrophe alone: Workers' benefits, States' Rights. Only plural nouns ending in s do this.

Answer 2

A possessive noun is singular if it references one person or thing and is plural if it references more than one person or thing. To make a singular noun possessive, typically add an apostrophe followed by "s" (e.g., dog's bone). To make a plural noun possessive, usually add an apostrophe after the "s" (e.g., dogs' bones).