Q. How would you write the first parenthetic instance of an acronym or initialism when the first defined use is plural? For example, if the first use of the term “part number” happens to be plural, would the initial declaration of the initialism be “(PNs)” or “(PN)”?
A. If you can’t rephrase the first mention to use the singular “part number,” then the initialism should be plural: “part numbers (PNs).” The usual assumption is that readers introduced to the singular “PN” will figure out what “PNs” means if that form occurs later on in the same document. It seems fair enough to expect the reverse—that readers will understand the singular from the plural.