You have to publish at least 3 chapters. You can publish how many you want, unless the rules of a certain contest state otherwise (e.g. the Halloween Contest is usually a short story contest = 3-5 chapters). Many authors do not publish completed stories for contests (neither did I, both times), but continue the stories after the contest has ended and the winners have been announced.
Episode does rate contest stories not really by what we as readers think, but by retention (reader engagement) and rubic score (choices, directing, etc). There is a whole posting on this topic in the “Explain” section in case you are interested in learning more about this 
I hope this helped and you’ll enjoy the entries and maybe find some new favorites to keep up with after the contest as well.