Answer
1.2 million electrons.
Work Step by Step
To make the jump, each electron must be provided with 0.72 eV, the energy gap. Divide the energy of the 820-keV photon by the energy gap to find the maximum number of electrons that can be excited into the conduction band.
$$N=\frac{hf}{E_g}=\frac{830000eV}{0.72eV}=1.2\times10^6$$
