If you want to get up to the Inner Army post, you'll have to go through both the Outer Army post and the Middle of Army posts first. Think of it as a succession through each and you have to meet requirements to get to the next stage. So it'll be 5 posts on the Outer Army, 10 posts on Middle of Army, and then 15 posts for Inner Army if you're going in alone.

I hope that makes sense.