whether or not you need an ISBN for your ebook depends on your distribution plans. If you are distributing your ebook only through your website via download, you probably don’t need an ISBN. But if you plan to sell your ebook through other resellers, you’ll have to find out what each website requires.
Amazon.com does not require an ISBN if you are publishing content with Kindle. The website says, “An ISBN (International Standard Book Number) is not required to publish content with Kindle Direct Publishing. Once your content is published on the KDP web site, Amazon.com will assign it a 10-digit ASIN (Amazon Standard Identification Number), which is unique to the eBook, and is an identification number for the Kindle Book on Amazon.com. If you already have an ISBN for your eBook, you’ll be able to enter it during the publishing process. Do *not* use an ISBN for the print book edition.”