Actually - you can have as many Kindles (or other devices such as Kindle for PC, iPhone, iTouch etc) as you want on your account. The rights holder to the books (usually the publisher) can set a license limit to how many devices each book can be downloaded to, this is usually 5 but sometimes different.
You can put a Kindle on your account, download books to it, and then remove the Kindle from the account. The books will stay on the Kindle until the user removes them but they do use one of the licenses.
If the Kindles remain registered on the account they can purchase books.