Answer
5 electrons.
Work Step by Step
The upward electric force balances the weight. The electric field is the potential difference divided by the plate separation.
$$qE=ne\frac{V}{d}=mg$$
$$n=\frac{dmg}{eV}$$
$$n=\frac{(0.010m)(2.8\times10^{-15}kg)(9.80m/s^2)}{(1.60\times10^{-19}C)(340V)}$$
$$n=5$$