I know Holy Grail and cards like that are twice regardless of how many multiples you have in your deck. But are cards like Carcasses, “.... may be used for two rounds.” Able to be used multiple times if the player has 3 in deck? I would say yes based on this, from the REG, “If an Artifact or Site has a usage limit that is “per game” or with no specified end, it is discarded after the last
use. If an Artifact or Site with a “per game” usage limit is played after the limit has been reached, it is immediately discarded.” Carcasses says may be used for two rounds, not twice per game so I would argue having 3 Carcasses allows 6 rounds of CBN discard.
