It turns out that is you are only publishing an ebook on Amazon and nowhere else you do not need to buy an ISBN as they assign their on code.
If you are publishing an ebook on other platforms as well, you’ll need an ISBN for each book you publish.
Some self-publishing ebook conversion companies offer to provide you with an ISBN as part of the package but if you wish to change things later and no longer go through them, you’ll have to buy one later.
There are two registering bodies for ISBN – in the UK it is Nielsen and they now co-ordinate with the US company Bowker. All ISBN’s are internationally recognised so you can go with Nielsen and still go worldwide.
You can buy one ISBN but it’s cheaper to buy a few at a time. You can go through middle men or you can go straight to Nielson or Bowker sites as all ISBN’s originate with those companies anyway.
When you register an ISBN you fill in their forms and then they are registered.