Do not use an apostrophe in the plural of any numbers.
Oksana skated nearly perfect figure 8s.
The 1920s are known as the Jazz Age.
Italicize the letter and use regular type for the -s ending.
Two large Ps were painted on the door.
He received two Ds for the first time in his life.
To avoid misreading, you may use an apostrophe with some letters: two A’s in biology.
Do not use an apostrophe to pluralize an abbreviation.
Harriet has thirty DVDs on her desk.
Generally, omit the apostrophe to form the plural of words mentioned as words. If the word is italicized, the -s ending appears in roman (regular) type.
We’ve heard enough maybes.
Words mentioned as words may instead appear in quotation marks. When you choose this option, use the apostrophe.
We’ve heard enough “maybe’s.”