[Yossar]

I build temples in every fort and more as necessary. I don't really have a formula. You want at least enough to keep all of your provinces in your dominion. If that isn't the case build more temples. Being in enemy dominion increases unrest. Other than that, dominion increases the stats of your pretender and your prophet. Build temples near the area you plan to attack with them to increase your power. There are many other bonuses to having a high dominion but those vary depending on your nation and pretender. High dominion is always good but you have to keep a balance between spending money to speed up expansion and spending money to increase dominion. That's one of the main aspects of the game and it's hard to say exactly how to do it.