In Texas y’all is plural. You don’t say y’all to one person.
Absolutely right. Y'all is plural. If, say, a Texan -- and I assume all native Southerners -- asks a person, "How are y'all doing?" it is understood that you are asking not only about the person you are addressing, but his family as well. This assumes, naturally, that you know this person pretty well. If not, then a simple, "How're you doing?" is used.
I'd be interested in hearing from other Southerners if this is standard practice for them, too.