I also had trouble locating the Sample Points dialog - after using GIMP-2.9 for maybe two years, I was still grumbling to myself about “why doesn’t GIMP have Sample Points”. I was about to file a bug report requesting same, but fortunately thought to ask another person if they were already available “somewhere”, which of course they were. Oddly enough, that other person was looking for some other feature, which also turned out to already be available. And we both learned that consulting the manual can be very helpful.
Regarding the number of Sample Points, already in GIMP-2.9 (I don’t know about 2.8) one can add more than four sample points to the image window, but only four are displayed in the Sample Points dialog.
@prokoudine - would adding more Sample Points to the dialog be as simple as adding additional replications of existing code? Well, also provision for a vertical scrollbar would be needed. I did a quick code search, but couldn’t find the actual code for the Sample Points dialog, that somehow limits the number of points to 4.