There are a total of 100 possible expansions on the map. You start off with 6, so you earn the other 94 through the three ways of getting them.
You get 35 city expansions (though Orcs), and if you complete 206 provinces, you'll have another 33. For a total of 68 plus your starting 6 gives you 74. If you want to fill in the rest with 24 premium expansions, it will cost you 24,600 diamonds. Or you could wait until more content is added or keep fighting through provinces. They didn't make it easy to get all 100, or cheap. You get enough in game to make it through, you may need to buy a couple to make it through Dwarves, but it don't see any reason to pay over $200 to make sure you fill the entire board.