thou is the singular intimate. (compare the tu- form, in french).

You could be singular or plural, but it was always formal. (compare the vous- form, in french)

Knowing this puts a whole new spin on things like the KJV, since as moderns we hear "thou" and think "fancy old timey speech! very formal!" and it is exactly the opposite.

Quakers/Friends chose the thou- form as the preferred form to address everyone; this was part of their scandalous behavior at the time, because it was heard as being entirely improper. (For their part, Quakers figure we're all equal before God, so why pay too much attention to social status? -- and that's not a bad point, really!)

I was taught Biblical Hebrew by an Australian scholar who learned hers in the south of the US, and I picked up from her the habit of translating the second plural as "y'all" :-) You can of course do the same with Greek. For some reason I preferred "y'all" to the more Australian "youse."

I don't know for “you”, but the French « vous » (or e.g. вы, вие, etc.) are only formal in the singular form, otherwise they're just bog standard 2pp.