I number in pencil on the back, not the front. The front is the picture. The back is the contract: title, year, fraction, a blind stamp if I remember the press.
When a print sells I cross the number in the ledger by the sink. The ledger is a school exercise book. It has coffee on it. That feels more reliable than a shop plugin.
Twelve is enough for a picture to travel and not enough for it to become wallpaper.
People ask for thirteen. I say no, and then I offer a different crop of the same negative, which is a different picture, and that can be its own twelve.
The shelf in the studio holds the ones that are still mine. Dust is part of the display. I wipe it on Saturdays.