It really depends on how the images are used. If it is a photo book where the images are the main value of the book then you would need the Extended Licenses. If they are used as a cover or to occasionally illustrate something that is in the book then you can use the standard license in up to 500,000 copies.
Full license details here: http://wwwimages.adobe.com/content/dam/acom/en/legal/servicetou/Adobe-Stock-Additional-Ter ms_20160119.pdf
Thank you for your quick reply and the link.
This raises another question :
-If the ebook is made available free (lest's say in epub format) , and shared on different platforms, there is no way to check how many copies are download/shared. It would be possible to track it if it's download from my server, but on other servers(or shared on Facebook etc), it is impossible to track a DRM-free work, and it defeats the purpose.
-Like you said " "You may not cause or allow any Work to be reproduced more than 500,000 times in total" . In a lucky word, let's say the book is downloaded from iBooks server (free or for a fee), What am I expected to do? I wish I can buy it and use it any way I want with no worries in the future.
- I only see subscription models for standard license. How can I purchase an extended license subscription(is it negotiated per picture?).
I just found out from another answer :
Does Adobe Stock offer extended licenses allowing licensees to sell derivative products or services based on an image?
At the moment, Adobe Stock offers only standard licenses, allowing the licensee to use the images for illustration purposes only.Extended licenses are available via Fotolia - Sizes and Licenses
Of course it is not always possible to track exact copies. As long as there is reasonable attempts to stay within the limit I am sure you would be fine. However as you say it is sometimes best to be worry free so you can get Extended licenses to cover you. When you go to license an image you have the option of standard or extended.