The balanced chemical equation for the reduction of dichromate with sulphur dioxide is shown.
K2Cr2O7+3SO2+H2SO4→K2SO4+Cr2(SO4)3+H2O
In K2Cr2O7, the oxidation number of Cr is +6.
In Cr2(SO4)3, the oxidation number of Cr is +3.
The change in the oxidation number of each Cr atom is 3.