I had thought “all y’all” arose where “y’all” had come to be used universally, even when referring to a single person, so that “all y’all” became the new plural form. But that seems to be an oversimplification. See this discussion on Language Log which I think makes it clear that singular y’all is relatively rare but by no means unknown, and suggests that “all y’all” is used in different ways in different places.
The last time I queried an authoritative source on y’all (a waitress in a Waffle House in South Carolina), she was quite sure that “you” should be used for one person, “y’all” should be used for up to 5-6 people, and if you’re addressing more than that, it’s “all y’all”.
I had thought “all y’all” arose where “y’all” had come to be used universally, even when referring to a single person, so that “all y’all” became the new plural form. But that seems to be an oversimplification. See this discussion on Language Log which I think makes it clear that singular y’all is relatively rare but by no means unknown, and suggests that “all y’all” is used in different ways in different places.
The last time I queried an authoritative source on y’all (a waitress in a Waffle House in South Carolina), she was quite sure that “you” should be used for one person, “y’all” should be used for up to 5-6 people, and if you’re addressing more than that, it’s “all y’all”.