Is there a special term for Elemenstors like Char or Portent who can use Gears and summon Elemanifestations of any element? How common are they?

I believe there is but I don't remember the term right now. As for commoness, all Elemenstors can use any element they wish to but they just have a natural affinity for one usually. It is unheard of (at least from what I've read) to have one that has an affinity for them all. Elemenstors like Char and Portent were around for so long that they gained proficiency in all aspects of Elemanifestation.

certainly historians have come up with all sorts of names for what Harbinger Portent was, but he merely referred to himself as a "High Elemenstor".. it wasn't until the generation of the "Second Students" that elemenstors began taking nicknames based on their elemental affiliation.

Char Reyarteb actually never even took the title of "High Elemenstor" or "Dark Elemenstor"... merely being called an "elemenstor"

although certainly those other labels have been applied to Char