If a card in any zone other than the stack has an {X} in its mana cost, the value of {X} is treated as 0, even if the value of X is defined somewhere within its text.
May 3, 2019 · War of the Spark
If an effect instructs a player to pay an object’s mana cost that includes {X}, the value of X is treated as 0 unless the object is a spell on the stack. In that case, the value of X is the value chosen or determined for it as the spell was cast.
January 25, 2019 · Ravnica Allegiance
If an effect instructs a player to pay an object's mana cost that includes {X}, the value of X is treated as 0 unless the object is a spell on the stack. In that case, the value of X is the value chosen or determined for it as the spell was cast.
January 23, 2015 · Fate Reforged
If an effect instructs a player to pay an object’s mana cost that includes {X}, the value of X is treated as 0 unless the object is a spell on the stack. In that case, the value of X is the value chosen or determined for it as the spell was cast.
February 1, 2010 · Worldwake
All instances of X on an object have the same value at any given time.