Historical Rules · 20 November 2020
Rule 701.13b
Official source text from the 20 November 2020 Rules release
A player can’t mill a number of cards greater than the number of cards in their library. If given the choice to do so, they can’t choose to take that action. If instructed to do so, they mill as many as possible. Similarly, the player can't pay a cost that includes milling a number of cards greater than the number of cards in their library.