Assuming they are both at room temperature and atmospheric pressure, how does the number of molecules in 1 L of oxygen gas compare with the number of molecules in 1 L of carbon dioxide gas

They both have the same amount of molecules because they both have 1 liter. 22.4 liters = 1 mol, and 1 mol has 6.02x10^23 molecules. if you convert the 1 liter to mols and the mols to molecules, they both have the same amount of molecules.