Gandalf, Westward Voyager

While a spell with {X} in its mana cost is on the stack, use the value of X that was chosen when it was cast to determine its mana value.

You copy the spell you cast, not any of the revealed cards.

If the spell that's copied has an X whose value was determined as it was cast, the copy has the same value of X.

The triggered ability and the copy it creates will resolve before the spell that caused the ability to trigger.

The token that a resolving copy of a spell becomes isn't said to have been "created" and won't interact with abilities that care about tokens being created, such as that of Rosie Cotton of South Lane.

You create only one copy, no matter how many revealed cards share a card type with the spell you cast or how many card types those cards share with the spell you cast.