Chances are, you don't.  However, the answer to this isn't quite Y/N, unfortunately, so, let's dive into it.

You do not need an ISBN if:

•You are only publishing your eBook on Amazon,

•Or only on Amazon and Nook, or,

•Only on Amazon, Nook, or iBooks. (I'm not 100% sure about KoboBooks).

•None of these entities requires, uses, or even notices an ISBN.

Don't believe what people say, that "Amazon gives you an ISBN."  They don't. They assign an ASIN, an Amazon Sales Identification Number, which is indeed unique, but it's not an ISBN.

You do need an ISBN if:

•You're publishing your eBook on IngramSpark, Lightning Source, Smashwords, or,

•Pretty much any distributor. This is because distributors need to track orders and sales, and they do this via ISBN.

You need two (2) ISBNs if:

•you are publishing both an eBook and a print book, and,

•you're going to use an ISBN for the eBook.

•eBooks cannot reuse the ISBN from a print book.  

•Don't assume that you can use the print book's ISBN, particularly if you are going to publish that eBook at IngramSpark or another distributor.  You'll most likely run into trouble trying to do that.

Yes, there are scads of blogs, Reddits, you-name-it out there telling you that you do need an ISBN, or that you can reuse your ISBN that you used at Ingram at Amazon, yadda-yadda-yadda.  Be smart—buy one ISBN for each print version of your book. Now, "version" does not mean retailer—it means version. If you have one 6" x 9" paperback that you've published yourself, bought your own ISBN, put that on it, and uploaded it via IngramSpark, you only need one ISBN. But if you want to publish that same 6" x 9" paperback at three different places, say, Amazon's Createspace, B&N's NookPress and Ingram, you really should get three dfferent ISBNs, no matter what any airhead tells you.  Why? Well, if you use the same ISBN for all three, and somehow, both Amazon and Ingram, or Nookpress and Ingram, both end up with the same exact book, with the same exact ISBN, at the Books4Ever Bookstore, how does the bookstore know which distributor sent them the book, and which to pay for sales of the book?  That, right there, is the point of ISBNs—so that bookstores and warehouses know which book came from which publisher or distributor, and who gets PAID.

 

You can get your ISBNs at Bowker, which is located at https://www.myidentifiers.com/