Each player starts with one unlocked slot for selecting and activating Dragon Arts. In order to increase the number of slots available, you must discover the Dragon Art of Additional Slots. This increases the amount of Dragon Arts you can have activated and receive bonuses from at once. There are 5 Arts of this kind; discovering each of them will unlock an additional slot in the Dragon Bonuses section of the Dragon tab.
Note: in order to master the Dragon Art “3rd Additional Slot”, you will need both Dragon Points and Dragon Marks. Mastering the Dragon Arts that unlock the 4th and 5th Additional Slots will cost you Sapphires, but not Dragon Points or Dragon Marks.