Each time you press the spin button, the spinner will revolve and one spin is consumed. Spins are refilled over time and you will get one spin once in every 12 minutes. The spinner is full when you have 15 spins and no new spins are generated before you have used one or more of them. There is an indicator below the spin button showing how many spins you have now (for example, “7/15”).
Sometimes you can have over 15 spins (e.g. the indicator shows “23/15”). This is because you have been rewarded with extra spins (for instance, hitting extra spin sector, completing a task, claiming a map drop, or gaining a new player level).
If you are out of spins, you can buy more spins with gold bars. Just tap the spin button and the game will show you a pop-up for refilling your spins.