The Hydra token enters the battlefield as a 0/0 creature. Any abilities that modify or trigger on this event apply. After the token is on the battlefield but before any player can take actions, +1/+1 counters are put onto the token.

If the spell you cast has in its mana cost, the token gets only X counters, not twice X.

If X is 0, the 0/0 Hydra token will die immediately after the ability's done resolving unless something else is raising its toughness.

The value of X in Zaxara's last ability is the value of X of the spell you cast.

If an effect, such as that of Parallel Lives, causes Zaxara's ability to create multiple Hydra tokens, they each receive X +1/+1 counters.