I've never seen anything to indicate what actual battlefield losses were either. I have no idea if we're even in the ball park.
However, if you include all the artillery that retired because of running out of ammunition or any other reason not related to actual loss, then the numbers don't seem so inflated. I think there's a lot of abstraction in the cannon loss mechanism in the game as well, so the actual destroyed guns is probably larger than it should be while guns that have left the field is lower. Yeah, at the end a lot of guns aren't available in the game, but I don't have a problem with the numbers.
Actual battles may have had more cannons at the end because they kept some back as a reserve. You probably have to adjust the doctrines to achieve this in the game, as artillery likes to move forward to support the front lines. Real commanders probably used their artillery a bit differently than we use ours too.
Hook