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".
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.