I don't think so. Seating capacity at the Roman coliseum was between 50-80k. Still an amazing fact, but certainly not 385k!
Mannix does not cite a source for his seating capacity of 385,000. Circus Maximus was much larger than the Coliseum, 2,000 feet long and 400 feet wide, more like the Indianapolis Motor Speedway, which seats 250,000.
Wiki says 150,000 seating capacity for Circus Maximus.