If you call pthread_create in a master thread or main program it is easy. This sit eh total numbers of threads created, not currently active threads

Code:

int thread_count=0;
...................
if(pthread_create(.....) == 0) thread_count++;

If "everybody" creates a thread, then you need to create a static mutex then a static integer variable in shared memory, initialize it to zero. Everybody calling pthread_create does this:

Code:

wait on mutex
get mutex & add one to the global counter;
release the mutex

pthreads have these basic calls (plus a lot of others)

Code:

create & destroy a mutex
      pthread_mutex_init(),
      pthread_mutex_destroy()
      PTHREAD_MUTEX_INITIALIZER is a macro to create a static mutex - probably what you want.

Lock/unlock a mutex.      

      pthread_mutex_lock(),
      pthread_mutex_trylock(),
      pthread_mutex_unlock()

that lets you play with mutexes. You will have to pass the mutex address to all of your threads or they cannot use it.