Answer:
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 have the 10 main, and 4 more shown in the last season. In the manga, they have all of that, plus 4 for the sailor comets, sailor cosmos, and they reference a good handful of other planets sailors. They have even more in the musicals, but I don't know how many they added.