Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Do not use an apostrophe in certain situations
The apostrophe
Punctuation

Plural of numbers and abbreviations

Do not use an apostrophe in the plural of any numbers or abbreviations.

Oksana skated nearly perfect figure 8s.

The 1920s are known as the Jazz Age.

Marco earned two PhDs before his 30th birthday.

Plural of letters and words mentioned as words

Generally, do not use an apostrophe to form the plural of letters and words mentioned as words. If the letter or word is italicized, the -s ending appears in roman (regular) type.

We’ve heard enough maybe s.

Two large Ps were painted on the door.

Letters and words mentioned as words may also appear in quotation marks. When you choose this option, use the apostrophe.

We’ve heard enough “maybe’s.”

Two large “J’s” were painted on the door.

EXCEPTION: To avoid misreading, you may use an apostrophe to form the plural of lowercase letters and the capital letters A and I: two A’s in biology.