Supercell (Blue)

The cost value of X is determined at the time the card is declared to be played so that X targets can be declared, X resources can be paid, and the cost value for the purposes of effects (e.g. Frost Lock)

You must have a target to play Supercell for any X>0. E.g. You cannot play Supercell with X=4 if there are not at least 4 Hyper Driver targets.

You can still play Supercell if you declare X=0. Supercell will just create a Hyper Driver token with 0 steam counters in the arena. However, that Hyper Driver’s state-based triggered effect will trigger as it has no steam counters on it and will create a triggered-layer to be added to the stack. If that Hyper Driver still does not have at least one steam counter on it by the time the triggered effect resolves, it is destroyed and ceases to exist.

Each targeted hyper driver will gain X steam counters. Eg. If X =3 you will add 3 counters to each hyper driver for a total of 9 counters.