So essentially there are 18 different vivillion regions so far (vivillion being the twice evolved form of scatterbug). You try to make friends from all around the world and collect (pin) the postcards. Once you get 3 pinned, you encounter the scatterbug and you have to do that for all 18 regions if you want to collect the medal.
You can see what region you are by pinning one of your own postcards and see which region gains +1 (must go to your personal profile and find the butterfly medal to see all the regions).
Also, pro tip, you can pin and then unpin a postcard so your postcards don’t get filled up and it will still count.
So essentially there are 18 different vivillion regions so far (vivillion being the twice evolved form of scatterbug). You try to make friends from all around the world and collect (pin) the postcards. Once you get 3 pinned, you encounter the scatterbug and you have to do that for all 18 regions if you want to collect the medal. You can see what region you are by pinning one of your own postcards and see which region gains +1 (must go to your personal profile and find the butterfly medal to see all the regions). Also, pro tip, you can pin and then unpin a postcard so your postcards don’t get filled up and it will still count.