Sorcerer Supreme or Sorceress Supreme is a title granted to the "practitioner of the mystic or magic arts who has greater skills than all others or commands a greater portion of the ambient magical energies than any other organism on a given world or dimension". By definition, there can be only one Sorcerer Supreme per world at a time.Formerly, during the Trial of the Vishanti, Khalid Inshallah, Jules St. Thomas, "Screamer," Sen-Yu, Tareva, and Wai Chee Yee, along with at least four other unnamed mystics were also deemed worthy candidates for the office, chosen as most learned in the ways of sorcery by the Aged Genghis.
The Lord of the Vampires before Dracula rose to power, Varnae (once summoned to the Assemblage of Avatars, a gathering of the most powerful users of magic existent on Earth at the time, including the then-Sorceress Supreme Zhered-Na, by the first Sorcerer Supreme Agamotto to seek a solution for the conflicts between the Elder Gods, but little was achieved during the meeting and Agamotto himself grew frustrated by the limitations of humanity), one time while battling Doctor Strange, has claimed that he once sought the title of Sorcerer Supreme before he became a vampire.
The Sorcerer or Sorceress Supreme is functionally immortal, having the immortality conferred upon him after assuming. Such a being with the role does not age nor succumb to medical diseases, after a trial with Death, which you are forbidden to fight evoking the power of the Vishanti, (in which one does not truly destroy death itself, but truly accepts one's own mortality), though he can be killed; the Sorcerer or Sorceress Supreme has access to the near-unlimited power of the Vishanti, the Octessence, and various other Principalities, as well as granted possession of such powerful mystic items ("gifts") as the Eye of Agamotto, Orb of Agamotto, Wand of Watoomb and the Cloak of Levitation, along with "books of knowledge" to help the Sorcerer Supreme train for his or her quests, such as the Book of the Vishanti.
Despite numerous statements that the Sorcerer Supreme is only to defend one world, his duty is actually to the whole of the dimension the world is within; Doctor Strange was the Sorcerer Supreme of the Marvel Universe, yet was often, erroneously, called only “Earth’s Sorcerer Supreme,” likely used by writers and editors to limit his power as to not enable him to diffuse every negative situation he or other characters were confronted with, as stated once by Marvel editor Joe Quesada, who admitted the difficulty of placing Strange in a truly “dangerous” situation. While it is certainly possible to have more than just one master of the mystic arts in one dimension, only one of them can hold the title Sorcerer Supreme at a time. More supporting evidence of this can be given by the fact there is no other known Sorcerer Supreme on any of the planets not of Earth; for example, there are at least tens of thousands of alien worlds in the Marvel Universe, many known to have potent mystics, yet none were called the Sorcerer Supreme. Furthermore, the mysterious cosmic being the Stranger, after revealing himself to not be the Beyonder, while describing the high potential, power and uniqueness of Earth (including the manifestation of the Phoenix Force there, the repelling of such awesome beings as Galactus and the involvement of the Kree, Skrull and Shi'ar Empires there for so many times), says “the Sorcerer Supreme makes his home there” and when Strange was captured once during the days of the Defenders, Strange was said to be the Sorcerer Supreme of their “cosmos,” strongly implying that the Sorcerer Supreme is not limited to his home planet, but the entire universe he resides in, for a defender of one mere world out of perhaps millions in the universe would be negligible indeed to one of the most powerful cosmic entities in existence.
Marvel Database Sorcerer Supreme Article- http://marvel.wikia.com/wiki/Sorcerer_Supreme