Since "Race" usually has 13 episodes a season (which most reality series, like "Survivor" and "Apprentice" do), and the 11th episode will air next Tuesday (9/14), CBS will air a two-hour season finale on 9/21.
Most likely this two-hour episode is actually episodes 12 and 13 combined into one extra long episode.
Here's CBS' lineup on September 21st:
8:00 Big Brother 5 (season finale)
9:00 Amazing Race 5 (season finale; two hours)