How to partially specialize a template function for a specific template with int paramater

I can't find another question which seems to answer this.

I have a template defined as follows:

template <int N> classA;

//the body of class A is irrelevant for this problem really.

now I have a second class template:

template<typename T> classX
{
 public:
  static const unsigned number;
}

with a default implementation:

template<typename T> classX<T>::number = sizeof(T);

I however want to have a "specialization" for classX if used with classA. But for all/any template parameters N. How to I do that?

thank you for your reply :)