Dunnigan's a smart guy, and he doesn't know, either. But he's trying to make is that surrendering is very dangerous.
There's a factor for any soldier intent on surrender needs to consider: it's very dangerous. The ground between contending forces is patrolled or observed by BOTH sides. That ground is likely to be mined & swept by automatic weapons & mortars.
The best way to survive is therefore to wait for the enemy to come to you. But there's a catch: Do not attempt to surrender to the enemy's assault troops (his first echelon). These men have a mission to consider, they have probably taken casualties, and they aren't very well equipped to take prisoners. If you pose any threat to them, or their mission YOU ARE DEAD.
So there's truly no way to know how many prisoners actually survive the "surrender process".
Another factor is that there is not a clear, distinct moment when a battle is over, and anyone surrendering too soon might find themselves gunned down accidentally or from the last spasm of rage, fear, and uncertainty as the battle winds down. I think that was depicted in "Saving Private Ryan."