Rule 901.9

Any time the active player has priority and the stack is empty, but only during a main phase of their turn, that player may roll the planar die. Taking this action costs a player an amount of mana equal to the number of times they have previously taken this action on that turn. This is a special action and doesn’t use the stack. Note that this number won’t be equal to the number of times the player has rolled the planar die that turn if an effect has caused the player to roll the planar die that turn. (See rule 115.2g.)

Find this number in the current Rules