Number of molecules in a volume of $4\, cm ^{3}$ of a perfect monoatomic gas at some temperature $T$ and at a pressure of $2\, cm$ of mercury is close to $?$
(Given, mean kinetic energy of a molecule (at $T$ ) is $4 \times 10^{-14}$ erg, $g=980\, cm / s ^{2}$, density of mercury $=13.6\, g / cm ^{3}$)