A lot of people confuse plurals and possessives in their writing, using apostrophes incorrectly. Here’s how it works:
When you want to express that there is more than one of something, you use the plural form of the noun (e.g., two keys, three elephants, a roomful of Williamses). Usually, this is done by adding s or es to the end of the noun (except for irregulars like children, feet, etc.). There are no apostrophes in plurals!
If you want to show possession, you add an apostrophe and an s to the end of the word, e.g., the dog’s bowl, my mom’s recipe, the Pope’s hat. The exception is if the noun already ends in s – then you just add an apostrophe at the end: Chris’ car.
The basic rule is that if a noun ends in s, to make it plural, add es. To make it possessive, add an apostrophe at the end.
What if it’s both? Say you want to express that the teacher “belongs to” (possessive) many students (plural). First, make the noun plural: students.
Because the plural form ends in s (remember, irregulars don’t), you will put just an apostrophe at the end: students’. So: “the students’ teacher gave them homework.”
One dress, many dresses. The (single) dress’ collar; The (several) dresses’ collars.
One (person named) Jones, several Joneses. The (single) Jones’ account; the (whole family) Joneses’ accounts.
I hope that clears it up!