E.g. Regice/ Regirock Key, Challenge Keys - are they one per cartridge?
Depends on the key
If your using the iceberg key or the iron key you will not be able to use them again but as for the easy normal or hard mode keys you will be able to continue using them again and again. also you will be able to change your city by keys as well (the level keys and the city keys are reusable)