Re: Who to call "Sensei"?
Technically speaking, every dojo has a Sensei, and that can be only one person, the other ones teaching if s/he is not in the dojo at the moment being more correctly instructors but not the Sensei.
However, less technically, there is only one Sensei in the whole of the world and time. Raising this Sensei from its immemorial slumber is why one should train. I don't know how this Sensei behaves once awakened (in my case certainly it hasn't, though I am prodding in that direction on a daily basis), but it may be said that, most likely, won't use any words. Normally, this type of entities speak by hints & cues (this because they use no grammar - their grammar being made of names only and no verbs, articles, or adverbs).