Answer
760 electrons.
Work Step by Step
Let the two spheres have charge $q.$ From Coulomb's law,
$F = k \frac{|q_1 q_2|}{r^2} = k \frac{q^2}{r^2} $
$\implies q = \sqrt{4\pi \epsilon_{0} Fr^2} = \sqrt{4\pi \epsilon_{0} (3.33 \times 10^{-21}) (0.2)^2} = 1.217 \times 10^{-16} \ C$
The quantization of charge means that for integral values of $n$, $q = ne$
$\implies n = q/e = \frac{1.217 \times 10^{-16}}{1.6 \times 10^{-19}} = 760 $
