Humphrey, John H., Roman Circuses: Arenas for Chariot Racing, University of California Press, 1986, p.3: link Humphrey gives a seating capacity estimate of 150,000 for the Circus Maximus.
Στάδιον, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
uchicago.edu
penelope.uchicago.edu
Contemporaneous references to seating capacity are implausibly high if their reference to loci is assumed to mean "seating places", but less so when taken as "per foot run of seating". Regarding the seating at the Circus Maximus, "the Notitia [says] that in the fourth century it had 385,000 loca [which] has been interpreted to mean that number of running feet of seats, which would accommodate about 200,000 spectators." See Samuel Ball Platner (as completed and revised by Thomas Ashby): A Topographical Dictionary of Ancient Rome, London: Oxford University Press, 1929.p. 119: Bill Thayer's website link
