Consider that I have a website with some subpages that I want to accelerate on mobile using AMP.
Do I have to maintain two versions of this subpages one with AMP e.g. using amp-img and one without (with img)?
Yes, note that some of the features of non-AMP HTML won't run on AMP pages like 3rd party Javascript. So on your AMP pages, you'll be doing validations. You also need to know how to link your non-AMP to your AMP pages.