Do I need an ISBN?

For a print book you effectively need an ISBN, but you have options:

• Free KDP ISBN — Amazon supplies one at no cost for a book sold through KDP. Easiest if you're Amazon-only, but it lists Amazon as publisher and can't be reused elsewhere. See Can I use a free KDP ISBN?
• Your own ISBN — bought from your country's ISBN agency (Bowker in the US; free in some countries). Use the same ISBN at KDP, IngramSpark, and other stores, with your own imprint as publisher.

For ebooks, Amazon assigns its own identifier (ASIN) and an ISBN is optional, though some other ebook stores want one. Remember that each format needs its own ISBN — see Does each format need a different ISBN?