I'm just curious whether if only Madara possesses the Rinnegan and if not how many are there? What are the chances that Sasuke can awaken his Rinnegan?
I just realized that the correct answer may be potentially wrong.
As we have seen thus far, Madara and Obito are the only 2 Uchiha to have used the Rinnegan. Aside from them, Sasuke is the only other Uchiha clan member that is still alive, whom hasn't awaken the Rinnegan yet.
With that said, Sasuke is potentially capable of awakening the Rinnegan any second now. Sasuke has bitten Karin many times. Karin is an Uzumaki, which is a distant relative of the Senju. That means Sasuke has Senju cells inside of him!
With the DNA requirements fulfilled, Sasuke can pull a quick one and activate Rinnegan stunning Madara, the Shinobi alliance, and all of us readers :)
Updated with known Rinnegan users: Madara, Obito, Sasuke, Sage of 6 Paths, Kaguya
So far as the manga goes, only Madara was seen to activate the Rinnegan. Even Nagato's rinnegan was given to him by Madara before his death.
And so all the rinnegan shown belongs to Madara.
Sasuke will only be able to activate rinnegan if he has Hashirama's cells with him. And i guess he currently don't have it.
As an updated answer, It turns out it was not as simple as we thought, though it took the Sage himself to explain it.
Based on the Musings of Hagoromo (the Sage of Six paths) He inherited His rinnegan from his mother, which was then split to each of his children, Indra and Ashura. It is the combination of their chakras that result in the rinnegan (Indra + Asura = Hagoromo). The brothers however had a Feud, which somehow caused their Unique chakras to reincarnate into their decedents. Coincidentally, they happen to be Uchiha and Senju respectively, which is where the initial confusion happened with only need Uchiha and Senju DNA. This is shown best with Danzo, who had Uchiha and Senju DNA for Years, possibly around the time of Narutos birth, but does not have any Rinnegan eyes in that collection. Eventually, the Chakras reincarnated into Madara and Hashirama. Madara obtained the Rinnegan because he was Indras reincarnation and therefor had Indras charkra, and then he took on Asuras chakra from Hashirama, who was Asuras Reincarnation. This mix (Asura and Indra) becomes the Sage of Six paths, with the Rinnegan.
Madara then gave both of his eyes to Nagato, since Nagato could not naturally obtain them. Rinnegan appear to work like all other Dojutsu in that the eyes themselves retain their form even after transplanting, as Nagato could never undo them, they were permanently rinnegan. After Nagatos death, Obito took his eyes. Obito is unable to awaken the Rinnegan naturally because he is not Indras reincarnation, and so is still missing Indras chakra despite having Uchiha and Asura's charkas. he had to implant the Rinnegan, which he could only implant one of due to their power.
This boils down to meaning that Only Madara and Hashirama could obtain the Rinnegan from the older characters, And the current generation of Reincarnations, Naruto and Sasuke as well. Of course, if someone could implant Sasukes eyes and implement Narutos DNA, they could also obtain the rinnegan eventually.