The molar volume of hydrogen gas at standard temperature and pressure is 22.4 liters. Standard temperature and pressure means a temperature of 0 degrees Celsius and a pressure of 1 atmosphere.
If the gas is not at standard temperature and pressure, then the molar volume is different. Molar volume is defined as the volume that 1 mole of a gas occupies at standard temperature and pressure. It is 22.4 liters for all gases, not just hydrogen, composed of single atoms or molecules. It does not apply to a mixture of gases. The rule is based on Avogadro's law, which states that equal volumes of gases at the same temperature and pressure contain an equal number of molecules.