Team scoring in Team battle format counts the top four players in a team. See more about what Team battle format is and how to set it up here.
As such you are always recommended to have at least 4 players in a Team. However, you may play with 1, 2 or 3 players - but be aware that your Team may be disadvantaged in the scoring.
The maximum team size you may enter online is 14 players. However an arbiter may manually change this by moving players into a single team, so again, there is no hard limit.