In the latest chapters of the Naruto manga (673 and 674), why does Sasuke possess only one Rinnegan and not both?
For starters: Sasuke's Rinnegan is not the same as Kaguya's/Madara's Rinne Sharingan (not to be confused with Madara's Rinnegan, I'm talking about the one in the middle of the forehead of both of them).
- Sasuke's Rinnegan has 6 tomoe, Rinne Sharingan has (at least) 9.
- Sasuke's Rinnegan is blue, Rinne Sharingan is red.
The reason Sasuke only has the Rinnegan in one eye was not mentioned or explained in either the series or the data book. So we can only speculate.
In no particular order, here are my speculations:
- Hagoromo only gave Sasuke enough chakra to awaken the Rinnegan in one eye.
- That's the best Sasuke can do without fusing Asura's chakra.
- It's the cool factor.
Sasuke's Rinnegan is exactly the same Rinnegan of Kaguya's 3rd eye and Madara's 3rd eye. Since, Madara's 3rd eye and Kaguya's 3rd eye are one and the same, this means that there is only one such eye.
This Rinnegan with Sharingan tomoe has always been a one eye case from the beginning. It is very different from Madara's Rinnegan, which is same as Hagoromo's pair of eyes.