[kergonath]

Two of them, just like in French or Spanish: a standard one and a polite form for people you don’t know. Social rank does not factor in this.

[User23]

I was familiar with my teachers and they still got addressed as “sie” exactly because a teacher socially outranks a student.

[kergonath]

Not really. Even the children of vips (however you want to measure social status, either wealth, political power, or whatever; it’s not really rare to outrank teachers socially) say “sie” to their teachers or to strangers.