<Tr> <Td_colspan="2"> List of Legends of Tomorrow episodes </Td> </Tr> <Tr> <Th> Preceded by </Th> <Td> "Invasion!" </Td> </Tr> <P> "Crisis on Earth - X" is the fourth annual Arrowverse crossover event, featuring episodes of the live - action television series Supergirl, Arrow, The Flash, and Legends of Tomorrow on The CW . The crossover began on November 27, 2017, with Supergirl and Arrow, and concluded on November 28, with The Flash and Legends of Tomorrow . In "Crisis on Earth - X", Barry Allen and Iris West's friends come to Central City for their wedding, only for the proceedings to be interrupted by interlopers from analogous universe of Earth - X where World War II was won by the Axis Powers . </P> <P> Development for a crossover of the four series began in December 2016 after the release of the previous crossover, "Invasion!". The premise and title of the crossover were revealed in September 2017 as production on the episodes began; elements from the animated web series Freedom Fighters: The Ray were part of the crossover, including the live - action appearance of Ray Terrill (The Ray), the Freedom Fighters, and the New Reichsmen . </P>

How many crisis on earth x are there
find me the text answering this question