No specific number of players is required to play "Truth or Dare" online. The number of players doesn't make any difference as to how the game will be played.
Anyone can join in an online "Truth or Dare" game, which can be played in large groups or even between just two players. The players should share interests and be of about the same age so that they have a common understanding of the questions or the dares. Players also need to select specific categories or environments before they can proceed. The kind of questions or dares depend on the chosen rating which can range from PG-13 to R or even X.