Traditionally, it is six, but TMF Hangman allows the game to go on and on. I believe the record is 20 guesses (including correct and wrong guesses), hence only 6 letters not guessed, when the puzzle was finally solved. 😛
But there is another problem. Suppose the person who won the last game and is running the current game goes offline and is missing in action for a long time. I believe we decided that, 48 hours after a guess, anybody could post, "Guess has gone unanswered for 48 hours, so I an starting a new game."