How did Danzo acquire all those Sharingan in his arm? I was just wondering, how did Danzo get all those Sharingan eyes in his arm? Is it from the people that Itachi and Tobi killed that night?
Danzo acquired his right arm from Shin Uchiha with the help of Orochimaru. This is found in Naruto Gaiden: The Seventh Hokage Manga.


  Orochimaru did another implant onto Danzō taking Shin Uchiha's
  Sharingan-embedded arm and placing that, alongside some of Hashirama
  Senju's cells, onto that of Danzō's.




(Shin Uchiha's Left Arm)

More on Shin Uchiha:


  Shin was one of Orochimaru's experimental test subjects and followers,
  and possessed a unique genetic make-up that allowed for tissue and
  organ transplantation with absolutely no chance of rejection.
  Together they performed various experiments, cultivating clones from
  Shin's teeth and nerves in order to uncover many secrets of genetics.
  At some point Shin became intrigued by the Uchiha clan, especially
  Itachi Uchiha. As such, he began to imitate the Uchiha: adopting their
  surname and customary clothing, as well as numerous Sharingan which
  have been implanted all over his body. His Sharingan-embedded right
  arm was harvested by Orochimaru and transplanted to Danzō Shimura
  along with some of Hashirama Senju's cells. Some time after this,
  Shin left Orochimaru's side and took most of his clones with him.