Bluebulb Projects' The Measure of Things presents:
21,000 is about two-fifths the number of Seats in New Yankee Stadium.
In other words, it's 0.41760 times the count of Seats in New Yankee Stadium, and the count of Seats in New Yankee Stadium is 2.39460 times that amount.
(2014 figures for 2009 Stadium; seating figures only) (a.k.a. Yankee Stadium, a.k.a. The Stadium, a.k.a. The House that George Built) (South Bronx, New York City, New York)