From the looks of it the "D"(imension?) Quincy is extensively using it (his armor is covered with black holes, perhaps portals) and he currently is in Kurotsuchi's laboratory, plus he seems to be the only one to freely manipulate them (like the moment he interrupted Hashvalt's and Bazz-B's quarrel.
Not quite, there can be multiple people holding the same letter/shrift, like Bach and Uryu, some of the SR girls also had letters that some males had, from what I remember. From the looks of it, the Old Man Gunner might also hold quite a high letter, seeing as he was the only SR shown to fluidly transform in and out of his Holy Form during heated battle. In other words, there can be even more than 10 individuals above Bazz-B.
It's probably also safe to assume that all SR of "J" and above can probably enter into their Complete Holy Forms.