Hi
Only one will count per stack. All abilities effects are not cumulative.
But for example you will want one sharpshooter per division in a stack, as this ability applies just to a unit (division) and not the stack. But only one sharpshooter per division is enough.
To see each ability effect "range" (self, unit, stack, only when commanding the the stack...) see this list
http://www.leqg.org/doc/aacw/listability.php5
Regards