You can only use 9 Dragons in each Brawl battle (up to 12 with VIP status), and after all Dragons have been defeated, your time in the Brawl comes to an end. You need to wait some time to be able to take part in another Brawl battle, which is initially 15 minutes but increases with each new Brawl up to 6 hours. You can also spend Gems to skip the wait time and immediately start a new Brawl battle.