Beautiful image. I look forward to the final map.
Generally most people (and map generators) like to draw borders around terrains to start with, and then chop those up if there is room.
The number of terrains can depend on how "large" a map you want this to be. Consider the number of nations in the eras, and in an average game how many provinces would each nation get before being forced into combat. The game considers small/medium/large maps to be 10/15/20 proves per player. If we look at early era its 24 nations. 21 land and 3 water. If they all played in a small game then the map would need to be about 240 provs, 210 land and 30 water. Here are some general numbers to consider...
http://www.dom3minions.com/RandomizedMaps.htm
All of that is very general of course. All maps of all sizes are welcome.
If you want the games map editor to guess neighbors then it "sees" red no-transparency borders easiest. I like to set the borders to red, get an initial map, then change the borders to something more comfortable to look at. The editor also seems to be abit smarter with fat borders (which I hate) so for big maps it might be worthwhile to make a second image and an "edge" tool to add red to both sides of the red borders before running it thru the map editor. It can save alot of tweaking the neighbors later.
By the way, an edging tool in a paint program can often be set for a range of color then told to edge the water, the mountains, the plains. It can give a start on provinces. And avoid working in jpeg. Thats only for displaying the final product.