Formula One has been ratcheting up the number of races on its calendar for decades. It ran with ten or fewer races through the 1950s and into the late 60s, then started climbing into the teens until it plateaued around 16 races throughout the 80s and 90s. But since then it's been climbing: 17 races after the turn of the millennium, 18 in 2004 and 19 in 2005. The year before last it reached a record 20 races, and now it's back up there again.
2015 Mexican Grand Prix