Complex sentence: As before, front rows were reserved for senators but additional seats were built for the equites; all were built in stone and accounted for approximately two thirds of Circus' total seating capacity of about 150,000.
Split into simpler sentences: As before, front rows were reserved for senators; additional seats were built for the equites. All of these were built in stone, and accounted for approximately two thirds of Circus' total seating capacity of about 150,000.