According to Reedsy. For anyone who doesn't know, Reedsy is a site/company that specializes in helping authors to publish. In addition to a number of for-a-fee services, they also offer some free services and templates, as well as periodic e-mails offering advice. They have now put out a blog explaining when, where, and why you need your own ISBN, and when you don't. https://blog.reedsy.com/guide/isbn-number/