The SSH model describes states of electrons in a polyacetylene chain, which is modeled as a lattice with two orbitals per site. Now, in many articles it is claimed that in the ground state, half of all the energy eigenstates (those with non-positive energies) will be filled due to the Pauli exclusion principle.
This means, the number of electrons is half the total number of sites.
But, does not every atom contribute a pi electron which can delocalize over the system? Then, the number of electrons will be equal to the number of oribitals, and the ground state will be a fully filled band.