I've heard differently on this question from various sources, so I'd like to know what would actually happen in the following situation:
I cast three copies of
Pyretic Ritual and a
Manamorphose (making at least one blue mana), then cast
Empty the Warrens, leaving one blue mana floating. My opponent responds to the Storm trigger with
Voidslime (my opponent's first spell of the turn) to counter it, but I counter Voidslime with
Dispel. Neither of us has any more responses, so the Storm trigger resolves. How many copies are made: four, since I cast four spells before I cast Empty the Warrens, or six, because six non-Empty the Warrens spells were cast before the Storm trigger from Empty the Warrens resolved?
That is, is the number of copies that will be created by a Storm trigger fixed when the trigger goes on the stack, or is it only determined when the trigger resolves?