One copy! A single copy of the book is destroyed and physically inaccessible.
The same effect would be if a hoarder purchased the book and never sold it or showed it to another soul. Same effect if a toddler purchased it, dropped it in their spaghetti bowl, and tore it apart.
But a copy of a book is a copy of a book! Books were designed for multiple copies being printed. Perhaps we recall Gutenberg and that whole controversy of easy printing from a press. So, the thing getting people all in a tizzy is that a single copy of a book that has, perhaps, become rare in 2026, a single copy that languished on a shelf somewhere, is being purchased and consumed.
Honestly, folks, books are really in no danger here. If you know any rare books that are irreplaceable, and with only one extant copy, just buy it yourself. For God's sake! Not rocket science.
Don't you think that if the books that Anthropic orders from New Zealand were available (online) anywhere else, they would order them from there rather than from New Zealand? So yes, I would say there's a pretty big chance that those books are really rare, as those are apparently the only copies you can buy online.
The actual reason to get angry about this is copyright law, which would rather have all surviving copies of a (niche, old, rare but still covered by copyright) book in private hands and inaccessible to the public, rather than make it public domain.