Answer is: twelve-electron.
Half reaction: Cr₂O₇²⁻ + 14H⁺ → 2Cr + 7H₂O
Oxidation number of chromium in dichromate is +6, because there are two chromium cations, altogether oxidation number of chromium is +12.
Oxidation number of chromium metal is 0.
From +12 to 0, 12 electrons is needed for reduction.
