Wizard's Spellbook

If your result is 20, you may cast the copies without paying their mana costs. You cannot pay their regular mana costs or any alternative costs the copies have. You may pay any additional costs and must pay mandatory additional costs. You may cast the copies in any order you choose.

Dice used must have equally likely outcomes and the roll must be fair. Although physical dice are recommended, digital substitutes are allowed, provided they have the same number of equally likely outcomes as specified in the original roll instruction.

An ability that tells you to roll a die will also specify what to do with the result of that roll. Most often, this is in the form of a “results table” in the card text.

If you control multiple copies of Wizard's Spellbook, each one tracks the cards it has exiled separately.

In all cases, any copies you don't cast cease to exist after the ability is finished resolving.

Tournament events have more specific rules regarding dice and die-rolling. For more information, please see the most recent version of the Magic Tournament Rules at https://wpn.wizards.com/en/document/magic-gathering-tournament-rules.