What does "spin degeneracy" mean in this context?
I'm not a solid-state chemist, but I think the meaning seems reasonably clear from context to me. &quot;Spin degeneracy&quot; here means that each energy level is capable of holding a spin-up electron as well as  a spin-down electron, i.e. in each orbital there are two different spin states which are degenerate (some would call these spin orbitals). Thus, $N$ atoms form $N$ different energy levels which $N$ electrons need to be distributed between. Because of this degeneracy, only the lowest-energy $N/2$ energy levels are actually populated: each energy level has $2$ electrons.