Apostrophe Is for Possessive, Not Plural

“I’m going over to Cajsa’s (house)!”
but “How many Cajsas are there here?”

An apostrophe is never used to form the plural of a family name: “The Jeffersons live here” (not “Jefferson’s”). With names such as Waters or Rogers, consider rewording to avoid the awkwardness of “Waterses” or “Rogerses.”

Capital letters used as words, abbreviations that contain no interior periods, and numerals used as nouns form the plural by adding s.

the three Rs
the 1990s
IRAs
vol., vols.
URLs
ed., eds.

The possessive of most singular nouns is formed by adding an apostrophe and an s, and the possessive of plural nouns by adding an apostrophe only.

the horse’s mouth
the bass’s stripe
the puppies’ paws
children’s literature
a herd of sheep’s mysterious disappearance

The general rule covers most proper nouns, including names ending in s, x, or z, in both their singular and plural forms, as well as letters and numbers

Kansas’s legislature
Chicago’s lakefront
Burns’s poems
Marx’s theories

One thing to remember is that pronunciation rules. When we speak, we say “Jesus’s contemporaries.” But “For Jesus’ sake!” or “For goodness’ sake!”