Ok i think i got it ! In fact, the dpi of an image is only a “tag” in the digital file. This tag is used to display (or print) the image in a certain way; the quality of the image itself is not affected by this tag and the tag can be changed later, depending on the type of display needed.
Then, indeed, a single product is enought to manage all the licences.
Ouf !
Thanks for explanations and your time spent here.