correct me if I am wrong: since any gas has (under sam temp. and pres.) the same amount of molecules
Same number of molecules in SAME volume. Please - be precise in what you write, you are leaving out details as if they were not important and in effect your statements become incorrect or ambiguous. That in turn means they need to be corrected as we have no way of knowing whether you just left them by mistake or you still have no idea what you are writing about - that's one of the reasons why we don't move forward.
and x and z have both 2 molecules, z must be 100, too, since x is 100 as well, or?
You are confusing things.
X and Z don't have "two molecules". As you said somewhere earlier, quite correctly, we don't know number of molecules of x nor z. What we do know is that:
a. one molecule of Y reacts with 2 molecules of X and produce 2 molecules of Z (that in turn means n Y molecules reacts with 2n X molecules producing 2n Z molecules)
b. per Avogadro's hypothesis under same conditions identical volumes of gas contain identical numbers of molecules.
What conclusion about volumes of reacting gases can you draw from these two facts? Can you calculate what volume of X will react with 10 mL of Y, and what volume of Z will be produced?