There are no permanent caps, as long as we keep getting new chapters. The expansion added with chapter 20 requires 620 completed provinces, which is 40 more than the one from chapter 19, so the one added when we get chapter 21 should also require at least 40 more completed provinces, or 660 total. However, it could end up being 50 more, or 670 total, based on the increases over the last couple of chapters. the one added with chapter 18 needed 25 more, the one added with chapter 19 needed 30 more, and then chapter 20 was 40 more. So if you are not at 660-670 yet, you have a goal to reach before we get chapter 21 in about another 4 or 5 months.
Now, the actual cost to scout never has a cap because the number of provinces you can scout is as unlimited as the storage space that the servers contains. Every single completed scout slightly increases the coin cost of every other unscouted province and only completing an Advanced Scouts research reduces that back down, just for it to slowly increase again with each scout after that.