You can target an artifact with indestructible. It won’t be destroyed, and you’ll still create X Gremlin tokens.

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.

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.