Get the amount of classes that inherit from another class

I looking for a way to get the amount of classes that inherit from another class.

class Movie {

}

I want to get number of extends

class Jaws extends Movie {

}

class Airplane extends Movie  {

}
class Braveheart extends Movie  {

}
class RockyMovie extends Movie  {

}

as you can see in this example, there are 4 extended classes

Is there a way to get this number?