Have you ever had somebody tell you that y'all is a singular reference, and all y'all is actually plural? If so, politely inform them that they are mistaken. It is a common misconception. I've heard this myth pretty often myself, primarily by people who don't actually use y'all in their common speaking. While I am technically a Southerner, being from Alabama originally, I don't count myself as one who uses y'all. Nonetheless, I have family in the South going back many, many generations who would be shocked at the inference that y'all meant anything other than you all (plural).
I've been called to task on this before, and so I had better cover my bases. There does appear to be a small minority of people, perhaps primarily in some locations in Texas (though not in areas any of my clan has ever lived), who insist on using y'all with reference to one person with no implicit reference to absent persons. English being a language as it is, there's really no way to prevent deviations from the norm, but this usage is not standard and is actually quite confusing.
Frequently I think that this is actually a misinterpretation on the part of others who do not comprehend an implied reference to other individuals not actually present. I know I've been by myself and have been addressed as y'all several times, but the inference is always, you and your kin, family, clan, folks, etc....
I think Wikipedia actually characterizes the controversy best:
While y'all is generally used in the Southern United States as the plural form of "you" a scant but vocal minority argue that the term can be used in the singular. Adding confusion to this issue is that observers attempting to judge usage may witness a single person addressed as y'all if the speaker implies in the reference other persons not present: "Did y'all [you and others] have dinner yet?"Well, there it is. If you insist on using y'all as a singular form of address, you are merely part of a "scant but vocal minority".
It has been argued by one linguist that the singular y'all is in reality a polite form of address, corresponding to 'vous' in French, 'usted' in Spanish, and 'Sie' in German.
And a few have noted what this linguist states in the following quote:That y'all or you-all cannot have primarily singular reference ... is a cardinal article of faith in the South. ... Nevertheless, it has been questioned very often , and with a considerable showing of evidence. Ninety-nine times out of a hundred, to be sure, you-all indicates a plural, implicit if not explicit, and thus means, when addressed to a single person, 'you and your folks' or the like, but the hundredth time it is impossible to discover any such extension of meaning.
– H.L. Mencken, , 1948, p.337
As for all y'all, I've personally only heard this used when a large group is being addressed. Perhaps it might correspond to a specific linguistic characteristic such as the Greek Dual.