When trying to find out how many artillery rounds were fired in a particular battle, you also have to figure out how many pieces were actually employed in the fighting. Artillery was kept in reserve and may or may not have been employed. The Austerlitz figure of 50,000 with 139 field pieces employed and firing leads to an expenditure of about 360 rounds per piece, which is very high. I agree that is too many.
However, having the number of pieces actually engaged might help. Just an idea.