Some people claim you must use a different ISBN for epub and mobi but I never have. I only use separate ISBNs for print and digital.
However, as I said above, you can use the same ISBN for the print format at different "stores" such as Amazon and Ingram. Another example is if you decide to hire a local printer to do 500 copies of your print edition so you can hawk them at book fairs; you'd use the same print ISBN. It doesn't matter who prints the book. The ISBN covers the edition and the format.
I may do a second edition of my only nonfiction title, adding a substantial amount of new text, a new introduction, etc. If I do that, I'll need new ebook and print ISBNs.
I also assigned an ISBN to my free novella for joining my newsletter, even though that "book" only appears in ebook format. And I applied for copyright for the novella, too.