multithreading (openMP) - how many parallel threads

I wonder whether more than 8 threads can run concurrently on a hardware with 8 cores. If so, using openMP to parallelize N calculations, I could create chunks of size, say, N/8, and in each thread further fork into (N/8)/8 threads, and maybe still more? How do things happen when I nested parallelize? do I still have 8 available threads for the nested parallel? Thanks!!