every cell in the phase space of volume $h^3$ may host up to 2 electrons with opposite spin directions. (here $h$ is the Planck constant.)
• Thanks @CosmasZachos. I understand the part related to the Pauli exclusion principle. But maybe since I haven't studied quantum statistical mechanics, I don't exactly understand why this granularity happens and why each cell's volume equals $h^3$. However I guess that this can be related to the uncertainty principle. – apadana Nov 3 '17 at 16:42