This amount of ballista's was combined with a archery research level of 9 and a attack hero with a stat of 131. If you have a low level attack hero i would suggest you increase the amount of ballist's between 50% to 65% to be cautious.

I am currently working towards level 9 however 6708 ballistas does fail against level 9.

I will try to update this when i find out last information.

If you have any questions feel free to ask..