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.
July 3, 2020 · Core Set 2021
rule 701.17b not present in this release
May 3, 2019 · War of the Spark
If a player is instructed to scry 0, no scry event occurs. Abilities that trigger whenever a player scries won’t trigger.
January 25, 2019 · Ravnica Allegiance
If a player is instructed to scry 0, no scry event occurs. Abilities that trigger whenever a player scries won't trigger.
April 27, 2018 · Dominaria
If a player is instructed to scry 0, no scry event occurs. Abilities that trigger whenever a player scries won’t trigger.
September 30, 2016 · Kaladesh
If a player is searching a hidden zone for cards with a stated quality, such as a card with a certain card type or color, that player isn’t required to find some or all of those cards even if they’re present in that zone.
September 30, 2011 · Innistrad
To untap a permanent, rotate it back to the upright position from a sideways position. Only tapped permanents can be untapped.