It is possible to estimate the number of crystal lattice cells for a single nanoparticle and calculate the probability distribution for the fluctuating transitions to the conduction band. But it is not so simple and the result can be predicted a priori - as said Dr Cornwall, it will be negligibly small.
If you can estimate the size (volume) of the nano particle , then you can estimate the number of free electrons knowing the size of the Pb2O3 particle. One way to estimate the size of a nano particles is using TEM or SEM imaging.