With 176 provinces done ring 13 around your city gets impossible to scout and ring 12 very likely at around 220 provinces done.
So if you scout circle after circle, 11 rings are probably the maximum for a total of 264 provinces.
If you start to scout to the borders early you can get a couple more provinces.
edit: with 12 rings completed you have 180 neighbour city spots discovered and 210 with 13 rings completed, what is very unlikely, because therefore you need to start to scout in ring 13 very early.
The possible trading partners are always 200 (the 200 nearest cities around your own), so you don't need to worry about getting them, only thing you can do is discover them to be able to trade without fee.