This is the most frequently asked question from the eldest to the youngest. The answer for this is also very difficult. According to science we say that energy can neither be created or destroyed but can be transformed from one form to another. The same way if a person dies the death is only to the body which is perishable and not for the soul which is like the energy. The soul cannot be destroyed. It can only be transformed into another energy or soul. Until the period of complete transformation the souls are considered to be devils but they are not.
According to Hindu mythology we say that if a person dies naturally his soul either goes to heaven or hell as per his deeds in his life time. If a person does not die naturally that is if he is murdured or suicide then the soul will wander this earth till it reaches it time to go to heaven or hell. The soul that wanders is considered to be devil which is said to fufill its wishes by distrubing other human. But the answer lies in one own self. If one strongly beleives that there is no devil then it is not there. But we are firmly tied with our traditions and myths which will never allow us to think so
According to me, only one devil is there. That's you. Otherwise am not believe in devil strongly.