The apostrophe has three functions: To help indicate possession (boy’s), to mark contraction (it’s), and to convert a singular letter, number, or initialism to a plural. However, the mark has all but been relieved of duty in its third task.
One of the few categories in which apostrophes are still retained for plural usage is when plurals of letters are concerned. In expressing how many times a letter appears in a word, for example, one would write “There are five e’s in beekeeper”; it would be distracting to write “There are five es in beekeeper.” This style also applies to the expressions “Mind your p’s and q’s” and “Dot the i’s and cross the t’s.” (Note, however, that in these idiomatic uses, contrary to the previous example, the letters are not italicized to indicate that they are being employed to refer to themselves.)
However, legibility is not a concern when uppercase letters are concerned: No apostrophes are necessary in “She received three As, two Bs, and one C on her report card.” (Note that names of letter grades are not italicized.) But to avoid confusion, don’t start a sentence with “As” or “Is” to refer to more than one uppercase letter; the resemblance to the words As and Is will distract readers.
If plurals of both uppercase and lowercase letters are listed in reference to the alphabet, though, be consistent: “The T’s and r’s in his first signature differ from those in the second one.”
Apostrophes are unnecessary when referring to plurals of numbers. For example, the treatment of the number in “I printed three 5s on a piece of paper” is correct, though when one is referring to any other use of the number than the numeral itself, it is better to spell out the word for the number: “She gave him change in the form of three fives.”
When pluralizing a year, omit the apostrophe: “They came of age in the 1990s.” Use the mark with numbers only to indicate the possessive case, as in “Check out this list of 1990’s biggest hits” — “Check out this list of the biggest hits of 1990” would be better — or to truncate a designation of a decade, as in “They came of age in the ’90s.”
Until relatively late in the twentieth century, inserting periods after each letter in an initialism was customary (“F.B.I.”). Because placing a plural s immediately after the final period would be awkward (“It was as if there were two F.B.I.s”), an apostrophe was customarily inserted before the s (“It was as if there were two F.B.I.’s”) — not an ideal solution, but better than the alternative.
However, now that these periods are almost universally considered obsolete (a few publications, most notably the New York Times, are holdouts), the apostrophe is superfluous and considered incorrect: “It was as if there were two FBIs.” (The Times, for the record, omits periods in acronyms, a series of letters that, unlike initialisms, are pronounced as words.)