An electron state has spin number s = ⁠ 1 / 2 ⁠, consequently m s will be + ⁠ 1 / 2 ⁠ ("spin up") or - ⁠ 1 / 2 ⁠ "spin down" states. Since electron are fermions they obey the Pauli exclusion principle : each electron state must have different quantum numbers. Therefore every orbital will be occupied with at most two electrons, one for each spin state.