In the answer of the question it is said that the material has an even number of valence electrons, meaning it is an insulator, but this isn’t true right? when it is even it can still be all 3 possibilities right?
Yes, you are right: an even number of electrons per unit cell is not enough by itself to conclude that a material is an insulator.
The intended argument has two steps. First, 1+7=8 valence electrons per NaCl unit cell, so completely filled bands are possible. This is only a necessary condition: with an even number of electrons, the material could still be a metal if bands overlap.
The second step is the optical information: NaCl is transparent, so there are no low-energy electronic transitions in the visible range. That tells us the Fermi level lies in a large band gap, and that is why we conclude NaCl is an insulator.
I clarified the model answer to make this distinction explicit.