no way to change that, unfortunately; since the number to take is the standard number of civ on the actual map size. And as said in the 1.10 patch, Rhye will not change that.
I would suggest then just removing these quest events. On a standard map you are NEVER going to have 17 cities - even 10 or so is a huge empire that is difficult to keep alive.
I wouldn't say never. I'm pretty sure I've had had that many and been stable before on a number of occasions. That said, only the largest of large empires is going to have a chance to pull the quest off.

