Plurals of Symbols, Letters, Numbers, and Years
Use 's to indicate the plural of letters, signs, or symbols spoken as such, or for words referred to as words when s alone would be confusing. Note the use of italics with the inflectional ending in roman type for words, letters, and numbers but not for symbols and signs. (See also 8.7.5, Punctuation, Apostrophe, Using Apostrophes to Form Plurals.)
He uses too many and's.
All of the capital P’s should be underlined.
Please use +’s to indicate a positive result.
Note: If the symbol can be easily expressed using words, this is preferred:
Please use plus signs to indicate positive results.
Do not use an apostrophe to form the plural of numerals (including years).
during the 1920s
a woman in her 50s