Rampaging Yao Guai

Vigilance, trample
This creature enters with X +1/+1 counters on it.
When this creature enters, destroy any number of target artifacts and/or enchantments with total mana value X or less.

Fallout

Vigilance, trample
Rampaging Yao Guai enters the battlefield with X +1/+1 counters on it.
When Rampaging Yao Guai enters the battlefield, destroy any number of target artifacts and/or enchantments with total mana value X or less.

A token that's not a copy of something else normally won't have a mana cost. Anything without a mana cost normally has a mana value of 0.

If a permanent has {X} in its mana cost, X is 0 when determining its mana value.