One SPSN for one format
A paperback, hardback, EPUB, PDF and audiobook are different formats. Apply for a separate SPSN for each format that you publish separately.
The title and many other details may be the same, but each public page must clearly show the format its number belongs to.
Where to display it
In a printed book, place the SPSN on the copyright page or another page containing publication details. In an e-book, place it in the copyright or publication-information section and add it to the book’s listing details where possible.
Use other identifiers when needed
A bookseller, distributor, library or legal-deposit service may require an ISBN. Check before publishing and use both numbers when needed.