Since this is a hot topic right now I'm going to explain why he's only got a single rinnegan, and why it's on his left side.
The chapter the sage gave Sasuke and Naruto their power he said "place your dominant hand out and I will grant you power". Naruto placed up his right, while Sasuke his left. This suggests that Sasuke is left handed. Humans also have a dominant eye as well. I'm guessing that's probably Sasuke's left eye.
A lot of people are saying he's only got one because the Juubi, and Kaguya did as well, but I don't think this is really the reason. The sage may have wanted to limit Sasuke's powers since he still is unsure about trusting him. So naturally he's going to only grant him a singlt rinnegan.
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