Potassium dichromate is a compound with the chemical formula K2Cr2O7. The oxidation number of potassium is +1, and the oxidation number of oxygen is -2. Let x be the oxidation number of chromium. Then we can set up the equation:

2(+1) + 2x + 7(-2) = 0

2 + 2x - 14 = 0

2x = +12

x = +6

Therefore, the oxidation number of chromium in potassium dichromate is +6.