Apostrophes are used to indicate contractions (can’t, don’t) and show possession (Bob’s car, Kate’s house). One of the most common misuses of an apostrophe happens in plurals, so follow these guidelines.
Plurals of lowercase letters. Use an apostrophe in instances like “There are two e’s in tree” and “Mind your p’s and q’s.” An apostrophe is used in this case to avoid awkward/confusing constructions.
Plurals of some uppercase letters. Do not use an apostrophe when saying “I got two Bs on my report card.” However, use “A’s” or “I’s” at the start of a sentence when referring to plurals of the letters “A” or “I” so that you avoid confusion with the words “As” and “Is.”
Plurals of numbers. Do not use an apostrophe when pluralizing numbers, such as “there are four 2s in my phone number.”
Plurals of years. Do not use apostrophes when referring to decades—for example, “the 1990s.” However, if you’re shortening the decade reference, use an apostrophe to indicate missing numbers: “the ’90s.” When showing possession, you should use an apostrophe—for example, “These are the 1980s’ greatest hits.”
Plurals of initialisms with no periods. When initialisms such as CIA, FBI or AARP are used as a plural, do not use an apostrophe: “It’s like there are two CIAs.” However, an apostrophe is necessary to show possession: “That is the AARP’s publication.”
— Adapted from “Plurals and Apostrophes (Mostly) Don’t Mix,” Mark Nichol, Daily Writing Tips.