Release the Gremlins

• WWizards of the Coast2017-02-09
  Official Ruling
  You can target an artifact with indestructible. It won’t be destroyed, and you’ll still create X Gremlin tokens.
• WWizards of the Coast2017-02-09
  Official Ruling
  If an artifact becomes an illegal target for Release the Gremlins, it won’t change the number of tokens created. However, if every target artifact becomes an illegal target, Release the Gremlins doesn’t resolve and no tokens are created.
• WWizards of the Coast2017-02-09
  Official Ruling
  The value of X is limited by the number of artifacts that you can target. You can’t target the same artifact more than once.

Rulings via Scryfall