Does the x in a spell's mana cost count as zero when copying the spell?

When a spell is copied, the copy duplicates the original object's copiable values, which include the choices made for X at the time of casting. This means the copy of a spell with X in its mana cost will have X equal to whatever value the original caster chose — not zero. This is governed by CR 706.10 and CR 601.2b.

Specifically, CR 706.10 states that when copying a spell on the stack, the copy uses the same choices (including the value of X) that were made for the original. So if you cast Fireball with X = 5, a copy of that spell also has X = 5 and deals the same amount of damage.

The only scenario where X is 0 on a copy is if X had never been assigned a value — for example, if a card somehow creates a copy of a spell before X is chosen. In normal gameplay this does not occur, because X is chosen as part of casting the spell (CR 601.2b), before the spell is placed on the stack and before any copy effects would apply.

Concrete example: You cast Fireball paying X = 4. Your opponent controls a permanent that triggers "when a player casts an instant, copy it." That copy of Fireball also has X = 4 and will deal 4 damage to the chosen targets — not 0.

Note that the controller of the copy does get to choose new targets for the copied spell (CR 706.10), but the value of X remains fixed to whatever the original caster chose.