Atoms can neither be created nor destroyed. This is the statement of law of conservation of mass.
Under similar conditions of temperature and pressure, equal volume of gases contain equal number of atoms.
As, $PV = nRT$ (Ideal gas equation)
$\Rightarrow\,\,\,\frac{P_{1}V_{1}}{P_{2}V_{2}}=\frac{n_{1}RT_{1}}{n_{2}RT_{2}} $
$\Rightarrow n_{1}=n_{2}$
$\left(\because P_{1},=P_{2}, V_{1}=V_{2}, T_{1}=T_{2}\right)$