This question already has an answer here:
Suppose I want to say that the (base 10) representation of a million contains 6 zeros, but for whatever reason, I want to use the symbol 0 rather than the word. Is there any rule on how to attach the "s" to make 0 plural? I have seen "0-s", "0's" and "0s" in various papers, but, sadly, it is not the case that these are always written in perfect English).
Of course, this issue is not unique to zeros. I might want to talk about how many times the number 42 appears in a book by Douglas Adams, or how many times the variable x appears in an equation. In general - how does one attach the suffix "s" to make a plural of something which is not a word?
Note: I am not sure if there is any rule about this, but nonexistence of such rule would also be useful to know about. Likewise, if making non-words plural is bad English, then that would be good to know too.