Second person is relatively unusual in literary use, particularly as published.* Of these even fewer maintain the second person throughout all or most of the narrative.
Now, of these again, some are quite clearly addressed to a particular character postulated by the narrative itself, perhaps as an identifiable character or perhaps not. Say for example the opening of The Perks of Being a Wallflower:
I am writing to you because she said you listen and understand and didn't try to sleep with that person at that party even though you could have.
As such, it is clearly second-person singular. By extension, one could just as well have a case that is clearly second-person plural.
Other cases may address the reader as the reader themselves (rather than as with Perks that we may not quite the same person to whom the letters are addressed). Sometimes the reader has qualities ascribed to them, e.g. in Bright Lights Big City:
You are not the kind of guy who would be at a place like this at this time of the morning. But here you are, and you cannot say that the terrain is entirely unfamiliar, although the details are fuzzy.
We're just one person. Also we're a guy! Also we're not the kind of guy who would be at a place like this at a time like this.
As such, while this is addressed to the reader, it also is addressed to a fiction, who we are asked to temporarily identify with. This is also the case with other point-of-view uses within other narrative arts, such as a point-of-view film, or a point-of-view computer game (Gone Home is an interesting case, in which the computer game medium is used solely to tell a story, and in this the "reader/player" is an American heterosexual woman in her early 20s even if someone like me is playing it, who has not been in my early 20s for some time, and never been any of the rest of those things).
And again, it is just about possible that when a writer conflates the reader with a particular fictional personality, that they might do so in the plural.
In most cases though the reader is no so firmly identified with a given personality, they remain merely, "the reader".
Now, one of the disadvantages of contemporary standard English is that after thou and ye became obsolete, you has been ambiguous as to number (many dialects have a way of being precise at least about the plural by retaining ye or introducing youse or you'all etc.)
However, one of the advantages of contemporary standard English, is this same ambiguity. In terms of "the reader" it's normal to refer to the reader in the first person as plural (note I used "we" above even when talking specifically about the singular) because when we talk about the reader in the first person we are talking about shared experience - what I and you and others all experience as the reader.
In terms of the reader as second person, we most often live in the ambiguity. After all, when I write something that isn't specifically addressed to a particular person then I am thinking both of the reader as a plurality of readers, but also as a hypothetical individual reader. And the ambiguity of English allows us to stay with that ambiguity easily.
If we address the reader as "reader" ("Reader, I married him." - Jane Eyre) we are using singular, but even this can be combined with other phrases that suggest plurality.
If we address the reader directly enough that number is unavoidable, then singular is probably most common, but plural would be far from unusual, and it's a matter of style and what best suits the needs of the writer. (In particular, if I want to address the range of experiences across my audience, then I must per force use the plural, but conversely addressing a singular reader affords a greater sense of intimacy).
But very often, we get to live in the ambiguity.
*More than a few comments from literary agents and editors would suggest that the percentage of works in the second person written is greater than the percentage published.