It is implied that there is a guardian for every single major celestial body in the universe. The number of Senshi is the same as this number, which is unknown by any human. But in the show, they...
They were Sailor Scouts before we ever saw them. Once they became Sailor Neptune and Uanus, they're effectively Sailor Scouts however we have no info on how Michelle became Sailor Neptune.
Yes there is a Sailor Pluto on Sailor Moon. Her English Human name is Trista Meioh and her Japanese Human name is Setsuna Meioh even though in Japan it would be written as Meioh Setsuna. She has...