Help talk:Citation Style 1/Perennial

Allow more than one ISBN
The ISBN field allows only one ISBN value by design. The ISBN is used to identify the particular copy of the work used as the source read by the editor per WP:SAYWHEREYOUGOTIT. There is no need to identify all versions of the work, and there is no guarantee that various versions have the same pagination or content or the same publication date.

If an editor insists on adding multiple ISBNs, then this can be done using the id parameter and ISBN magic linking. This sort of use will most likely fail Featured Article criteria.