1) I think only a few people know. Alad may not even know, as he was driven off by Sentient before the Warframe retrieved its Tenno, and when you fight him on Uranus he talks about his creation as being made of Tenno not Warframes. It was a big deal when Nora Night referred to the Tenno as dreamers in this last patch.

4) They waffle a bit on this. In some quests like The Second Dream, the Tenno is a lone actor. But in Survival missions, Lotus mentions causing a distraction for another Tenno, and Alad V was building a weapon out of Tenno (well, Warframe) corpses. I would guess they number in the hundreds, possibly thousands. Not a lot when you consider the entire settled universe.