Rule 901.15a

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. The planar deck can’t contain more phenomenon cards than twice the number of players in the game. Each card in the planar deck must have a different English name.