I don't keep an exact round by round count, but rather have a very good idea by how many boxes of ammo I shoot through any particular firearm, and how many rounds per box.
Much like when I was stumbling around the world in the old C-141. You counted the beers in the cooler before you went to the hotel, and then counted the ones left the next day. The difference was what you drank.
Same thing with the rounds. At some point you say "Huh, I've done a thousand [beers] rounds."