Two-Headed Giant matches consist of one game. All players from the two teams play in the same game.
Drawn games (games without a winner) do not count toward the one game. As long as match time allows, the match continues until a team has won a game.
This is a reference to different nature of Two-Headed Giant. The MTR isn’t the right place to explain the rules of Two-Headed Giant, they are contained in Magic Comprehensive Rules section 810.
Just like in regular matches where “best of three” really means “first to two wins”, “best of one” really means “first to one win”.