Historical Rules · 1 May 2012
Rule 901.14a
Official source text from the 1 May 2012 Rules release
As an alternative option, a Planechase game may be played with just a single communal planar deck. In that case, the number of cards in the planar deck must be at least forty or at least ten times the number of players in the game, whichever is smaller. Each card in the planar deck must have a different English name.