On DMM, there are currently 7 chapters for each nation except one (which is a newly revealed nation), with 5 maps per chapter except Chapter 7. Nation defense runs alongside dragon/coin dailies.

Nation requirements only apply to the girls on your 4 squads. You can still pick any helper team, and you don't have to change your helper team to fit the specific nation for use by allies.

Rewards include a gem for 3-medaling every map once, with nation-specific petals (blooming material) dropping on occasion, but then Nutaku doesn't have blooming system at the moment. Hence, there isn't much use running nation defense aside from the gem.