It's known that the number of states in a band is equal to the number of unit cells in crystal. Here is my problem (confusion with trivial concepts) Bloch function is a electronic state, or orbital. The number of orbitals in a band inside the first zone is equal to the number os units cells in crystal. each state can accommodate at most two electrons. So the max number of electrons that can occupy a single band is 2N But here is my stupid question: I know that a state can be occupied with two electrons with opposite spins, but why two? In my mind (i know that im thinking wrong) one state should be only occupied by one electron. Is like having N chairs for 2N people (N male and N female). We are saying that one chair can be occupied by one male and one female simultaneously . But one chair is made for one individual and not two individuals. Other question: If the Pauli exclusion principle would not be valid for electrons, then what is the max number of electrons that can occupy a single band? infinite?