It's about three-fifths the number of McDonald's Locations in the World.
In other words, 21,000 is 0.58 times the count of McDonald's Locations in the World, and the count of McDonald's Locations in the World is 1.7 times that amount.(2015 figures) (approximate)
Operating in 119 countries, there are currently 36,000 McDonald's locations worldwide. From 1955 thru 1993, many McDonald's locations advertised the number in billions of hamburgers sold on their external signs, but abandoned theses updates after the figure passed 100 billion.
It's about three-fifths the number of Seats in Fenway Park.
In other words, 21,000 is 0.56648 times the count of Seats in Fenway Park, and the count of Seats in Fenway Park is 1.7653 times that amount.(2013 figures; daytime capacity) (a.k.a. The Cathedral of Boston) (Boston, Massachusetts)
Fenway Park, the home of the Boston Red Sox baseball team, can accommodate 37,071 spectators when configured for daytime seating. Fenway Park holds the record for most consecutive sellout games, with a streak of 794 sellouts that ended in 2013. This figure is a dramatic turnaround from the attendance in 1965, which was at times as low as just 500 people.
It's about two times the number of Benches in Central Park.
In other words, the count of Benches in Central Park is 0.5 times 21,000.(Manhattan, New York City, New York)
Central Park — the 843-acre park located in the heart of New York City — contains about 9,000 park benches. The park is one of the most famous in the world and entertains over 35 million visitors each year.
It's about two times the number of Written Characters in the Chinese Language.
In other words, 21,000 is 2.245 times the count of Written Characters in the Chinese Language, and the count of Written Characters in the Chinese Language is 0.4454 times that amount.(zhōngwén, 中文) (Hànzì, 汉字/漢字; Shuowen Jiezi count; distinct characters)
According to the Han Dynasty dictionary, Shuowen Jiezi (說文解字), there are 9,353 distinct Han characters used to depict the Chinese language. Fluency in written Chinese requires mastery of 4,000 - 5,000 of these characters.
It's about two-fifths the number of Seats in New Yankee Stadium.
In other words, 21,000 is 0.4176 times the count of Seats in New Yankee Stadium, and the count of Seats in New Yankee Stadium is 2.3946 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)
New Yankee Stadium has a seating capacity of 50,287. The Stadium was built between 2006 and 2009 out of 11,000 pieces of Indiana limestone.
It's about two-and-a-half times the number of Blue Whales in the World.
In other words, the count of Blue Whales in the World is 0.4 times 21,000.(Balaenoptera musculus) (2002 estimates)
The worldwide population of blue whales (as of 2002) is estimated to be between 5,000 and 12,000. An estimated 378,200 blue whales were killed during the height of whaling in the late nineteenth and early twentieth centures
It's about two-fifths the number of Seats in The Colosseum.
In other words, the count of Seats in The Colosseum is 3 times 21,000.(a.k.a. The Coliseum, a.k.a. Flavian Amphitheatre, a.k.a. Amphitheatrum Flavium, a.k.a. Anfiteatro Flavio or Colosseo) (Rome) (estimated)
Built between 70 and 80 AD, The Colosseum was a central attraction of Rome, capable of seating 50,000 spectators for various events including plays, war reenactments, and gladiatorial matches. The structure was estimated to require 100,000 cu. m of stone to construct. It remains the largest amphitheater in the world.