Tap the + button that's next to where it shows how much Crystals you have on the upper side of the game or go to the in-game Store. There you may select how much Crystals you wish to buy.
OVERDOX
Tap the + button that's next to where it shows how much Crystals you have on the upper side of the game or go to the in-game Store. There you may select how much Crystals you wish to buy.