Since there is a fixed number of Amulets required to unlock the reward dragons in the Treasure Hunt Event, once you have collected all the Amulets, the event is considered complete and you can no longer take part. These Amulets can be spent at the Altar, where you can choose which reward dragon to unlock. If you have collected every Amulet, no new maps will appear and the event will be over. Additionally, any unspent Amulets will disappear once the event is over -- so don't save them for future Treasure Hunts!