Spellstutter Sprite

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2007-10-01
  Official Ruling
  The value of X needs to be determined both when the ability triggers (so you can choose a target) and again when the ability resolves (to check if that target is still legal). If the number of Faeries you control has decreased enough in that time to make the target illegal, Spellstutter Sprite’s ability won’t resolve (and the targeted spell will resolve as normal).

Rulings via Scryfall