How can I find out in Win64 how many processor cores there are in the system?

To get information about the number of processor cores in the system, you may use the Windows environment variable NUMBER_OF_PROCESSORS. Below is a fragment of C++ code where the WinAPI GetEnvironmentVariable method is used to extract and display the contents of this environment variable.

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

#define VARNAME TEXT("NUMBER_OF_PROCESSORS")
#define BUFSIZE 4096

int main()
{
  TCHAR buf[BUFSIZE];
  DWORD dwRet = GetEnvironmentVariable(VARNAME, buf, BUFSIZE);
  if (dwRet > 0 && dwRet < BUFSIZE)
    _tprintf(_T("Number of processors: %s\n"), buf);

  return 0;
}

References

1. MSDN library. GetEnvironmentVariable Function.
2. Discussion at social.msdn.microsoft.com. About the __cpuid on windows XP x64.