At STP (standard temperature and pressure), one mole of a gas occupies 22.41 liters of volume. Avogadro's number (6.022*10^23) is the number of molecules per mole of a substance, or one mole of anything really. This means there are 6.022*10^23 molecules of any gas with a volume of 22.41 L at STP.
At STP, Pressure = 1atm, temperature = 278K, R= 0.082 litre-atm-(mol-K) ^-1. V= 22.4L. Applying gas equation, PV=nRT. n = PV/RT = 1x22.4/0.082x278 = 1mole. Number of molecules of gas= moles x Avogadro's number = 1x6.02x10^23 = 6.02x10^23.
