Hmmm... there is somewhere in the forum a discution about that where it says something about that. I find it for you and then post the link.
Edit: I've found it here
Originally Posted by Konstantin Fomenko
For example, your army of Level 5 is in Wold Region. There are 500 total player armies and 200 cities in the Wold Region. Out of these 300 armies and 70 cities are close to your armies level and can be engaged. I won`t get into specific details - but our approach would get your player vs player going in these conditions with-in a few seconds with your nearby allied armies/other player`s nearby allied armies joining into this battle.