There is a limit to how many units of each type are most effective per group, called the State Based Damage Efficiency. Forum thread on SBDE goo.gl/HBY9fu

The first 8 infantry will function, and the rest are mostly along for the ride. They are hiding behind the first 8 with their hands in their pockets and trying not to get shot at, while taking credit for the accomplishments of the first 8.



Of course there is another aspect, which is damage absorption. When your stack of 30 infantry is attacked the first 8 return fire, but the damage done to the group is spread among all 30.

If 30 damage is done to a stack of 8 infantry, then 2 infantry are eliminated.

If 30 damage is done to a stack of 30, then each one loses one hit point.



So each method has advantages and disadvantages. I break the rules of SBDE when the situation calls for it.

If 30 damage is done to a stack of 30 infantry you still lose 2 infantry. That is why diversifying your stacks helps. If you have 15 infantry and 15 of another unit type like motorized infantry the damage is spread between the two unit types.
If the stack of 30 received less than 30 damage during the battle, you would lose 1 infantry and might be able to withdraw part of the stack after the battle.
If 15 damage is done to a stack of 15 or more infantry, the stack would lose 1 infantry.
If you receive 14 or less damage to a stack of 15 or more infantry in battle, the stack would have no losses.
If you receive 28 or less damage to a mixed stack of 15 infantry and 15 motorized infantry, you have no losses.
if you add a 3rd unit type, it might absorb some of the damage.

Better answer by Lawrence