C++11이상이라면 std::thread::hardware_concurrency를 보세요

//detect못 할 때는 0을 리턴합니다.
unsigned concurentThreadsSupported = std::thread::hardware_concurrency();

Win32:

SYSTEM_INFO sysinfo;
GetSystemInfo( &sysinfo );

numCPU = sysinfo.dwNumberOfProcessors;

Linux, Solaris, & AIX and Mac OS X (10.4 이상):

numCPU = sysconf( _SC_NPROCESSORS_ONLN );

FreeBSD, MacOS X, NetBSD, OpenBSD, etc.:

int mib[4];
size_t len = sizeof(numCPU); 

/* set the mib for hw.ncpu */
mib[0] = CTL_HW;
mib[1] = HW_AVAILCPU;  // 이게 안 된다면, HW_NCPU을 써 보세요;

/* get the number of CPUs from the system */
sysctl(mib, 2, &numCPU, &len, NULL, 0);

if( numCPU < 1 ) 
{
     mib[1] = HW_NCPU;
     sysctl( mib, 2, &numCPU, &len, NULL, 0 );

     if( numCPU < 1 )
     {
          numCPU = 1;
     }
}

HPUX:

numCPU = mpctl(MPC_GETNUMSPUS, NULL, NULL);

IRIX:

numCPU = sysconf( _SC_NPROC_ONLN );

Mac OS X (10.5 and newer) or iOS (any version) using Objective-C:

NSUInteger a = [[NSProcessInfo processInfo] processorCount];
NSUInteger b = [[NSProcessInfo processInfo] activeProcessorCount];