As much as possible
My idea of having a few more tags for countries is a bit different, than what I've seen so far. Although Euskadi, Colombia, etc seem historical and/or plausible, I would like to take another route.
Let's say if I play Germany and defaet the French, I should be able to demand that they create a border state (much like Vichy later) The name is actually of second importance to me. Even if the new state stays in the sphere of influence of France (possibly a vassal or satellite) it would weaken the French state enough to make it a viable option.
There could be a multitude of nations in the game, since I think every culture should have their right of a nation. Even it is implausible or ahistorical. Why, if I play Austria, do I need to hang on to these Croatians or Rhuthenians, or the two provinces with Ukrainians, which just give me high militancy and are a headache in the long run. Why can't I swap them (provided they are not cores) or if no-one wants 'em, or I don't want give them away, can make them independent?
Only the Austrian Empire alone could consist of:
1. Bohemia, 2. Bukovina, 3. Carinthia, 4. Carniola, 5. Dalmatia, 6. Galicia, 7. Küstenland, 8. Lower Austria, 9. Moravia, 10. Salzburg, 11. Silesia, 12. Styria, 13. Tyrol, 14. Upper Austria, 15. Vorarlberg;
16. Hungary proper 17. Croatia-Slavonia 18. Bosnia and Herzegovina
Plus on top of that, I would really argue, that, if I create a country, or satelite, I should be able to tell, which provinces they get, and which not.