Bluebulb Projects' The Measure of Things presents:
21,000 is about two-fifths the number of Seats in The Colosseum.
In other words, the count of Seats in The Colosseum is 3 times that amount.
(a.k.a. The Coliseum, a.k.a. Flavian Amphitheatre, a.k.a. Amphitheatrum Flavium
, a.k.a. Anfiteatro Flavio or Colosseo
) (Rome) (estimated)