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Depends on how jewed the government is when it comes to laws and how they are enforced. A high trust society with no diversity won't require many cells at all.

In the end, one has to figure out what the main purpose of prison is too. Is it to punish? Then why not just execute the prisoners and let God judge them.

If it's about rehabilitation, then you'd also need to make sure that each individual can be rehabilitated, otherwise it's a waste of time and money. Halden seems to be the best prison in the world for that, and even they only manage to rehabilitate 80% if the prisoners. The rest should be executed.

If we're talking isolation, were the prisoner is only a threat to one specific person, a gps tacker would be sufficient. The victim will set up geo fencing were the prisoner can move, if the prisoner goes outside of the designated zone or remove the tracker, that's an execution by cop.

If we had proper justice and gave the death penalty to those who deserved it, probably zero.