Letters, numbers, symbols, and words used as themselves are italicized or underlined. See Underlining or Italicizing Items that Name Themselves for more on this.
When these items are made plural, the plural is shown by adding apostrophe s to the underlined or italicized item. The apostrophe and s are not italicized or underlined.
Some authorities make acronyms or abbreviations plural by adding apostrophe s. Not all authorities recognize this rule.
Other authorities make a date that names itself plural by adding apostrophe s. Not all authorities recognize this rule.
These three instances are the only times in English when adding an apostrophe plus s makes something plural.
Since not all authorities recognize the last two instances, whether you choose to add an s or to add an apostrophe s, be consistent.
Examples: Don't forget to dot your i's.
(Letter as a letter)His 7's look like 2's.
(Number as number)His &'s look like 8's.
(Symbol as symbol)I find the thee's and thou's in older writing hard to follow.
(Words as words)Recognized by some authorities: He has IRA's in several different banks.
(Plural of acronym or abbreviation)The 1930's were called the Red Decade.
(Plural of year naming itself)Recognized more widely or traditionally: He has IRAs in several different banks.
(Plural of acronym or abbreviation)The 1930s were called the Red Decade.
(Plural of year naming itself)