If a creature on the battlefield has {X} in its mana cost, X is considered to be 0.

If Skyclave Apparition leaves the battlefield before its first ability resolves, the ability still exiles the target permanent.

If there's no exiled card when Skyclave Apparition leaves the battlefield (most likely because its first ability hasn't resolved yet), no player creates a token.

If Skyclave Apparition's first ability exiled more than one card owned by a single player, that player creates a token with power and toughness equal to the sum of those cards' mana values. If the first ability exiled cards owned by more than one player, each of those players creates a token with power and toughness equal to the sum of the mana values of all cards exiled by the first ability.