Calculating the E of a galvanic cell
Standard electrode potential can only be measured experimentally by comparing the half cells, in your case $\ce{Cr^{VI}/Cr^{III}}$ and $\ce{I2/I-}$ with a standard hydrogen electrode or a calomel electrode. The net standard electrode potential will be obtained by subtracting those of cathode-anode. You can find the standard electrode potential of most half cells in your textbook or online.
See in your reaction, two moles of $\ce{Cr^{VI}}$ turns to $\ce{Cr^{III}}$. So $n = 2 \times (6 - 3) = 6$. You can try out the same thing with the other half cell. The answer will be the same.