This is a very tough question to answer if you would want to be partially accurate at least. The reason why this is is that there is a big difference between the amount of rounds that were being transported to the Western front and the amount that were also actually fired. In any case, the best way to calculate this would be to calculate all the rounds that were transported there over the course period of the whole war and then calculate how many were left after the war. This gives you an idea how many rounds were fired.
