The Flavian Amphitheare was conceived by the Emporer Vespasian and completed by his son Titus in 80 AD. It was built to hold 70,000. The population of Ancient Rome is the subject of much debate about how to interpret the results of the census'. Did they include women and children? Did they include slaves? And so on. The most recent academic research seems to point to a number in the region of 500,000 but I wouldn't put any money on this figure.