Charges on spells?

[FoOta]

I don't know how to add a charge on a spell. I want to make a spell Like Earthen Shield in wow (Protects the target with an earthen shield, attacks to the unit heal the shielded target for 270. >>6<< charges.) How do u add that 6 charges to the spell? Can anyone please help me? Or even the spell Lightning Shield that has 3 charges and everytime someone attacks the unit, the attacked unit gets damage (can only occur 3 times, then the buff is removed).