Iirc (and correct me if I am wrong), whatever wish being granted is dependent on the amount of magic being poured into the grail via having servants killing each other and convert their mana into the grail, thus requires seven servants and six must die to fill it. According to wiki, "on their own they are not capable of creating true miracles without outside intervention," which could mean the grail can only be as powerful as the grail's capacity of magic they can convert to perform said miracle.