Here is a doc showing the attack a given number of troops will do.
It shows that one infantry will do 1.2 damage and 3 infantry will do 3.6 damage BUT 6 infantry will do 6.4 damage. As you group together infantry for the most part attack/defend damage will not go up linearly. As you get bigger and bigger stack the damage each additional troop does is lessened. Therefore attacking a group of 15 in 3 waves of 5 will have you win if morale is the same.
So essentially I should always use stacks of 5 of infantry, as there is no benefit and only merit of using anything above that?