The answer to this is 0. You should remember that it is not possible to create an abstract class because you do not have all of the right implementations to make it happen. The abstract class serves another purpose which is to make sure that you have a base for all of your subclasses.
This means that on its own, this is basically empty and unusable. You can use it in order to improve it or build on it. By doing these things, that is the only time when it can be useful for your needs. It is best that you do not try to create abstract classes anymore especially if you have tried it before as it will never work.