The variable is static long _number_of_calls in class InterfaceSupport, in file interfaceSupport.inline.hpp.

The place to write the variable is in macroTRACE_CALL, as below:

ifdef ASSERT
 #define TRACE_CALL(result_type, header) \
 InterfaceSupport::_number_of_calls++; \
 if (CountRuntimeCalls) { \
     static RuntimeHistogramElement* e = new RuntimeHistogramElement(#header); \
     if (e != NULL) e->increment_count(); \
 }
 #define TRACE_CALL(result_type, header) \
 /* do nothing */
 #endif // ASSERT

But can't find the usage of this variable, so assume it's a useless variable.