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?

Storm counts the number of spells cast before the spell with storm was cast. It will not count any spells that were cast while the storm trigger was on the stack. In your case, the storm trigger will make four copies of Empty the Warrens, since four spells were cast before Empty the Warrens was cast. It will not count the two spells that were cast with the storm trigger on the stack, since they were cast after Empty the Warrens was cast.

Nathan Long
Magic Rules NetRep