For general purpose calculation you may try following approach:
Air is a mixture of 21% O2 and 79% N2 (I am leaving 1% of Ar as it don't influence the result). Assuming that it behaves like ideal gas (which you can safely do as long as you are far from high pressures and low temperatures) you may treat thhis mixture as a ONE gas with molar mass equal to weighted average of molar masees of oxygen and nitrogen - so it will be
(21*32 + 79 * 28)/100 = 28.8
Now, you know molar mass and you know that under STP 1 mole of gas has volume of 22.4 liters. Use pV = nRT to calculate volume for your temperature (or use just V/T = const) and youare almost ready - you take 'molar mass' of air and you divide it by the 1 mole volume. Pronto!