There is a lot of empty space between gas molecules. The molecules are very far apart, relative to their sizes, and they neither attract nor repel each other much. So distances between molecules aren't affected by the type of molecules in the sample.
That implies that the number of molecules in a given container won't change, when you change only the type of gas filling the container. (This is one way to state Avogadro's Law).
Here is the same very small container, filled with dry air on the left, and wet air on the right. The drawings aren't to scale; the distance between molecules is much, much larger under normal conditions. Notice that in both pictures, there are 10 molecules in the container.