Two large circular discs separated by a distance of 0.01 m are connected to a battery via a switch as shown in the figure. Charged oil drops of density $900 \mathrm{~kg} \mathrm{~m}^{-3}$ are released through a tiny hole at the center of the top disc. Once some oil drops achieve terminal velocity, the switch is closed to apply a voltage of 200 V across the discs. As a result, an oil drop of radius $8 \times 10^{-7} \mathrm{~m}$ stops moving vertically and floats between the discs. The number of electrons present in this oil drop is $\_\_\_\_\_\_\_\_\_$ . (neglect the buoyancy force, take acceleration due to gravity $=10 \mathrm{~ms}^{-2}$ and charge on an electron $\left.(\mathrm{e})=1.6 \times 10^{-19} \mathrm{C}\right)$