can kratos have more than one account recovery code active at a time, e.g. one via email and one generated by the Admin UI? Or is it a single slot in the back-end database and the most recent request wins? Not a problem either way, I would just like to document it.

Do you mean per identity? Recovery codes are always tied to a specific flow. If you create one for an identity via the Admin API, but the user also starts recovery on their own, they get a different flow with a different code. If they request a new code in the flow, the first one will be invalidated.