That is not entirely correct. You don't need 15 nobles in a city. You need 15 nobles in a duchy that has a city to form a new realm.
You used to not have this restriction. It was added to reduce the number of realms since we had too many small realms that were inactive and semi-dead.
I just saw this post. So I was blocked from seceding all this while?!
I used to think it depends on number of nobles in a realm before you can secede.
