The typical way people track ammo is:
- fire a shot;
- erase total arrows, write new total;
This is why I’ve proposed something similar to this: don’t erase or tally after every shot; just keep track of how many rolls for missile attacks you’ve made. It probably won’t be many, so it should be easy to remember. When combat ends, roll that many d6s: that is how many missiles you have used. If you have the time, you can recover up to half of those missiles; the rest are broken or stuck. Deduct the arrows you can’t recover from your character sheet, all at once instead of every round.