Is there a limit to the number of extras for a movie?

Absolutely this.

If you make one of those movies an extra it won’t enjoy:
VPTs
Watched Status
or anything else Extras don’t need that Movies do.

You can’t put two versions in the same folder as the Extras.

The approved and tested method is to add the movie in the folder with the extras first. Then add ‘Versions’ outside that folder, in their own unique folder.
Merge or Split as required after that.

If you think you may have confused the order of things - it’s easy enough to Plex Dance the whole mess and try again.

When going for the Plex Dance Do-Over - anything having to do with this movie must be removed - Versions, Extras, the works. Scan, Empty Trash and Clean Bundles at that point will wipe out everything associated with this movie - except it’s watched status, if you can believe it (it’s unbelievable, but believe it anyway).