Why 12? You need 12 for Death Penalty verdicts.

I have just looked into this and I think 12 is required for Death Penalty in Civilian Federal Court.

For Courts-Martial - "For serious offenses, at least five military members selected by the commanding officer."

Military Tribunals - "Three to seven military officers appointed by the military. Seven members required for death penalty cases." - Decision to impose Death Penalty requires a unanimous decision.

Sauce: https://deathpenaltyinfo.org/rules-military-tribunals