There are 5 races in Art of Conquest: Humans, Dwarfs, Lichs, Rakans, and Sylphs. Currently only Humans, Dwarfs, and Lichs are playable.
How to switch races: Click on your territory's main stronghold (Castle for Humans, Engine for Dwarfs, or Pyramids for Lichs), then scroll to the right of the list of buildings at the bottom of the screen. There will be an option to switch races, provided your stronghold level and Rank is high enough. Switching races will cost a certain amount of Honor.