Rule: The plurals for capital letters and numbers used as nouns are not formed with apostrophes.
Example: She consulted with three M.D.s.
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
Example: the ’90s or the mid-’70s
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.