AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
Plurals of Symbols, Letters, Numbers, and Years
Use ’s to indicate the plural of letters or symbols 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 and letters but not for symbols (see 8.7.5, Using Apostrophes to Form Plurals).
All 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 a positive result.
Do not use an apostrophe to form the plural of numbers (including years).
during the 1940s
a man in his late 60s