The Storm triggered ability doesn't count the number of spells cast during the turn at the right time. It should count the number of spells already cast as it goes onto the stack, and create that many copies of the original spell when it resolves.
At the moment, the Storm ability is counting spells cast (along with creating copies) as it resolves. This means, for instance, that a
Counterspell targeting a
Grapeshot will counter the original spell but will cause an extra Storm copy to be generated (you can wait until the Storm ability resolves to cast the
Counterspell, but you shouldn't have to.)