How does Nagato create and use so many Rinnegan eyes? The corpses that Nagato uses for the Six Paths technique also possess Rinnegan eyes! How did he get so many Rinnegan eyes?

Is it that the corpse used for one of the six paths has a Rinnegan activated by default? Or is it a convention that Kishi has maintained just to picturize and show us the connectivity?

Of course, they aren't true eyes. 


   Even when Tobi possessed Rinnegan, his puppets (corpse he used) had both sharingan and Rinnegan! Now, from where did he bring 6 Sharingan eyes and 6 Rinnegans? Definitely they aren't real ones, What kind of Jutsu is that, that makes corpse possess eye technique?
Rinnegan occurs so rarely that the Sage of Six Paths was considered a myth, so those multiple Rinnegan's are surely not distinct ones. 

Nagato's bodies seemingly having their own Rinnegan is an ability provided by the Six Paths of Pain technique. According to Narutopedia:


  The Six Paths of Pain (ペイン六道, Pein Rikudō) is an Outer Path technique
  devised by Nagato after being crippled by Hanzō and rendered emaciated
  from the numerous chakra receivers embedded in his back by the Demonic
  Statue of the Outer Path. By piercing corpses with chakra receivers, a
  wielder of the Rinnegan can manipulate the bodies as though they are
  their own.


The article further says:


  Because of the fact that he was crippled, Nagato channelled the
  abilities of the Six Paths into six different bodies (one ability per
  body). The only common features shared by these bodies were the
  Rinnegan they gained from this technique, the chakra receivers each
  were pierced with and their bright orange hair, which had been dyed to
  match the Deva Path's natural hair colour.


In other words, the multiple Rinnegan's are like "projections" or copies of Nagato's own Rinnegan, transmitted through the chakra rods. This theory can be further confirmed based on the following: 


   In Tobi's version of the Six Paths of Pain technique, his six bodies received a Rinnegan and a Sharingan, and he had a Rinnegan and a Sharigan at the time.