14.8. Notes on Templates and static Members
What about static data members? Recall that, with a nontemplate class, one copy of each static data member is shared among all objects of the class, and the static data member must be initialized at file scope.
Each class-template specialization instantiated from a class template has its own copy of each static data member of the class template; all objects of that specialization share that one static data member. In addition, as with static data members of nontemplate classes, static data members of class-template specializations must be defined and, if necessary, initialized at file scope. Each class-template specialization gets its own copy of the class template's static member functions.