Avogadro's Law

Avogadro's law: equal volumes of gases at the same temperature and pressure hold equal numbers of molecules, so equal moles.

Two identical containers of CH₄ and O₂ at the same temperature and pressure hold the same number of molecules.

Same volume, temperature and pressure: same number of molecules

Not the same mass: O₂ ($32\ \mathrm{g/mol}$) weighs twice CH₄ ($16\ \mathrm{g/mol}$). Not the same atoms: CH₄ has 5 per molecule, O₂ only 2.