Historical Rules · 1 October 2009
Rule 806.10
Official source text from the 1 October 2009 Rules release
The Two-Headed Giant variant can also be played with equally sized teams of more than two players. Each team's starting life total is equal to 15 times the number of players on the team. (These variants are unofficially called Three-Headed Giant, Four-Headed Giant, and so on.)