Danzo only has 1 sharingan eye on his right side also he has some kind of special arm (his right arm also) his sharingan first shows on naruto shippuden episode 199 but it only shows for a few sec when he kills these assassins with his eye.
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This fight between Danzo and Sasuke happens in Naruto Shippuden Episode 209: Danzo's Right Arm. More information about Itachi comes out in this episode as well.
He Got it From Obito when he was on a mission when he was younger. Obito got crushed by a rock and gave kakashi his Sharingan as a gift before he died. Obito had the Sharingan because he was an Uchiha.
Episode 208
It hasn't happened in anime, only in manga.
He wasn't born with Sharingans. He collect Sharingans from the Uchiha Massacre and by the help of Orochimaru he implanted Sharingans in his hand and one eye along with First hokage cell for chakra.