You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t attack unless the controller chooses to pay the cost (a cost of zero is not automatically paid, so the controller can choose to not pay).

The ability only applies to attackers declared after it resolves. It will not add costs to any attackers already announced.

In the Two-Headed Giant format, you still only have to pay once per creature.