If a permanent has {X} in its mana cost, X is considered to be 0.
2019-01-25 • wotc
Tokens that aren't a copy of something else don't have a mana cost. Anything without a mana cost normally has a mana value of 0.
2019-01-25 • wotc
Kaya's last ability causes you to gain life equal to the number of cards the target player owns in exile, even if an effect causes her to deal more or less than that amount of damage to that player.