= State counting in a Bloch band
A crystal with $N$ <primitive unit cells> has $N$ distinct <wavevectors> in one <Brillouin zone>. For a rectangular periodic sample, $k_i=2\pi l_i/L_i$, and the product of the numbers of allowed values along the three directions equals the number of cells. Each orbital-derived <energy band> therefore holds $N$ states per <spin> orientation, or $2N$ with twofold spin degeneracy. This means two states per primitive cell per spin-degenerate band, independently of how many atoms the cell contains.
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