NOTE: Originally ""the supreme witch" is the world's queen of witches, or just the queen of the salem witches descendants. Another thing, why, silver bullets blessed, why blessed¡¡¡¡¡" Admin edited title for clarity.
Why not blessed? If you can accept the traditional folklore of silver's power over supernatural creatures then you should have no trouble accepting the legendary influence of religious rituals and relics.
As to the influence of The Supreme, I think it certain that her reign does not extend beyond the Salem descendants. We don't even know that there are any other witches in the world, but if there are then they are no more likely to recognize The Supreme's sovereignty than do Voodoo's practitioners.
It was never explained, but while it seems like Fiona Goode traveled the world as The Supreme Witch, Delphi Trust only covered North America. It seems odd, though, that no other witches came to the aid of or mentioned to the Salem witches that they were being decimated. So, it seems likely that the Salem witches were the only ones.
Fiona told the girls in the first episode that there was an alternative Coven on there field trip just before discovering Delphine. So I think that there are more than one Coven and each Coven can have its own Supreme, although a Coven can have more than one house as Cordilla said she would open more houses if she had to in the final episode before her final confrontation with her mother.
While I do believe that witches exist globally, I think that the concept of the Supreme is unique to the Salem descendants. After all, there is only one Supreme per generation, and having there be one on every continent complicates the line of succession a bit.
However, witches from other regions most likely have their own hierarchy in regards to leadership. They recognise the Supreme's authority, but they don't adhere to them like the Salem descendants do.
What most interests me about The Supreme is that in 'Coven' , we see that the mantle of Supremacy passes from one witch to the next, during which the former regent faces a variety of sudden, horrible illnesses ranging from cancer to diabetes and other things, almost like their mortality is catching up to them and all the years they were Supreme being immune to such things is now bestowing all the things they should have already had onto them. In 'Roanoke', however, we learn that the witch Scathach is the *first* Supreme according to Ryan Murphy, pre-dating the Salem witches by *centuries*, and is still alive and immortal after ALL THESE DECADES LATER - presumably having first made her presence known around the B.C. era!!! That places Scathach as being one of the oldest living beings in the AHS universe thus far next to the Devil from 'Asylum', which to me would signify that long, long before we see her in the 'Roanoke' season, she may have been the one creating the witchcraft bloodlines - a fact that may be supported by her extreme lust for hapless men, who she might have left with children that shared her supernatural abilities; thus, the first witches. We also learn that she was not originally from America, having been a stowaway on a ship that took her to the New World for as-of-yet-explained reasons. With that said, we also need to look at Delphi Trust, the organization that has hunted witches worldwide since *before* the dawn of Salem; at one point, Harrison Renard, the owner of the corporation, states that they did a great job of disposing of another coven with silver bullets prior to events seen in 'Coven', and in the last episode of the season Cordelia is putting pins on a map in places where other witches have been discovered. Thousands arrive to New Orleans after Cordelia reveals the existence of witches publicly on TV, yet none of them are aware of what a Supreme is, one young witch even asking "What's a Supreme?" before Queenie replies "You're looking at her" in regards to Cordelia. With all of this in mind, it seems that there are definitely other Supremes out there, but we don't know how many. For all we know, Scathach may be a disruption to the natural order, and because she is immortal and can't die, another 'Supreme' witch (or at least a witch that shares the same amount of powers as Scathach) must die in her place; this may be in part because of the Roanoke Colony sacrificing people to the old gods, who in turn have kept Scathach alive and prevented her status as Supreme from fading (a-la Fiona Goode style), so the natural order of the universe kills the closest thing to another Scathach as it can find. Ultimately, it seems that there are numerous witches that can be called a 'Supreme', but it seems that only the Salem witches use that title specifically.