The Apostrophe with Numbers, Letters and Abbreviations

Rule: The plurals for capital letters and numbers used as nouns are not formed with apostrophes.

Example: She consulted with three M.D.s.

BUT

Example: She went to three M.D.s’ offices.
The apostrophe is needed here to show plural possessive.

Example: She learned her ABCs.

Example: the 1990s

NOT

the 1990’s

Example: the ’90s or the mid-’70s

NOT

the ’90’s or the mid-’70’s

Example: She learned her times tables for 6s and 7s.

Exception: Use apostrophes with capital letters and numbers when the meaning would be unclear otherwise.

Example: Please dot your I’s.

You don’t mean Is.

Example: Ted could not distinguish between his 6’s and 0’s.

You don’t mean Os.

Source: Grammarbook