Why does Sasuke have 6 dots on his Rinnegan When looking at Sasuke's Rinnegan, I noticed that he had 6 dots in it (the same kind you find on a Sharingan).



But Madara didn't have this. His just had a few circles but nothing else.



Is there a reason for this?
The dots you're mentioning is actually called Tomoe, like you said, it is usually formed on Sharingan. Rinnegan are usually characterized by light purple irides with a ripple-pattern that spreads over the eyeball, a few well-known wielders are Pain (not really though), Nagato and Madara.

Back to the question, recall that Sasuke received half of Hagoromo's (aka Sage of Six Paths) chakra, thus awakened the Rinnegan. The Rinnegan Sasuke has is quite unique, it has 6 tomoe split between its first two ripples, this doesn't mean nothing, it indicates the Rinnegan is fully charged. What's the difference between a typical Rinnegan and a fully charged Rinnegan you ask? Well, it's simple. Fully charged Rinnegan wielder like Sasuke gains abilities typical Rinnegan wielders don't have. About the abilities, I bet you knew it already.

Sasuke cannot use the Rinnegan full abilities if he overuses it. At this point, the tomoe will disappear, preventing him from using its full power. Sasuke will keep his left eye closed until it is fully recharged. Sasuke is still able to use the powers of his Mangekyo Sharingan with his Rinnegan, but cannot use or form the Mangekyo in his other eye when it is recharging. (See the image, the Mangekyo doesn't form on his right eye when he loses Rinnegan's charge, and it is formed again when the Rinnegan is fully charged. This is also stated on Narutopedia.)

Here you can see it happen in the manga.