Rens Databook: Tenseigan

"The Tenseigan is a dōjutsu introduced in The Last: Naruto the Movie. The movie's novelisation explains that the Tenseigan can be acquired when an Ōtsutsuki is implanted with the Byakugan of a Hyūga: this combination of the two clans' chakra transforms the Byakugan into Tenseigan."

I pasted this directly from the wiki to clear a particular point up. People seem to have a misconception between the Dojutsu Tenseigan and the energy vessel known as "Hamura Ōtsutsuki's Tenseigan".

"Hamura made the energy vessel by removing all the Ōtsutsuki clan's Byakugan and clustering them together, a practice that continued in the centuries following Hamura's death."

People seem to interpret this as you can create the Tenseigan by harvesting tons of Byakugan. This is NOT the case. The Byakugan that make up the energy vessel are stored within a golden sphere that is made to resemble an eye, specifically the Tenseigan. This is not a real Tenseigan it's just an energy source with nice casing.

This is proven when Naruto and Hinata destroy the vessel and the Byakugan stored within the energy vessel disperse throughout the area and are subsequently lost into space.

The Tenseigan itself also likely requires you too have the Ōtsutsuki body specifically so you can handle the thing.

Main point:

This thing is not as easy to get as simply slaughter all the Hyuga and merge thousands of eyes or in Rens case just clone the eyes. This does though mean you can make an arc reactor from Byakugan for some reason, as the vessel powers everything on the moon.