You line the pit to prevent the effects of thermal expansion and contraction from damaging the pit over time. This is true for any pit, but, particularly for pits made from modules (such as brick or CMU's). The expansion and contraction of unlike materials will force the joints to fail. The use of un-mortared fire bricks (or other such dense material) will absorb enough heat to prevent the failure fo the external (more permanent) pit walls.
If you build from true clay brick, there is a small benefit to the use of the denser red brick, plus, it major league looks better. But, the benefit has a significant cost in dollars spent, and there is no benefit to poorly built brick, it will fail quickly.
Calculate the number of CMU blocks you would use, then figure out the volume of the cells, if you are going to fill each cell, you can easily figure that volume. It will be more than you think. I leave you to the math.