If I say:
I have to change my file names.
What does an English speaker understand?
- "I have to change names of a single file."
- "I have to change names of many files."
What is the rule here?
Are you sure they didn't mean
I have to update my filenames
According to here filename is a noun defined as
A name given to a computer file to distinguish it from other files, often containing an extension that classifies it by type.
So when they said they had to update their filenames, they may have had to go through a list of files on their computer and correctly rename each one.
There is no rule, but e.James has identified there is a clear implied plural here, and I agree that is how I would read it.
But the question is wrong really, it should say "how would you correcly write this sentence to be grammatically accurate and unambiguous".
If the single word "filenames" were used, that would be fine, and mean many names of many files.
If the words are separated, they are simply two nouns and need something to connect them - a possessive, in other words. Then you either have "file's names" or "files' names" to be grammatically accurate, and all ambiguity is lost. That's why grammar works!