In acidic medium, the dichromate ion is reduced to $Cr^{3+}$: $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$. This reaction involves the transfer of 6 electrons per mole of dichromate, so 6 Faradays of charge are required.