This question has already been raised.
/topic/at/359580/
If the number of creatures is no more than 10, then the damage from each of them is applied independently.
If the number of creatures is greater than 10, then the damage from 10 independent units is calculated, after which the total damage is multiplied by N/10, where N is the number of creatures in the stack.
Thank you for the information!
Although, in my opinion, this is a rather clumsy way to calculate damage :(
For example, the fact that for a stack of k*10 units, the damage will always be divided by k, and there are only (Dmax-Dmin+1)*10 possible values for damage for any stack. All this modulo modifiers, of course.