If you see the actual battlefield strewn with bodies, or the mass graves, or the field so covered with skulls that you can't avoid stepping on them, a million deaths has a million times more impact than just one.
I respectfully disagree with someone who has been involved on this forum far longer than me. Even if you walk a battlefied with a million casualties yourself, your mind becomes overloaded with suffering before you walk by a few hundred casualties, much less a million. We may understand a million in the abstract, but our emotional minds simply can't.
By the way, in my last post, I didn't mean to show any disrespect to the victims of 9/11. The comment by RJK hadn't posted when I was writing my post.
I lived in NYC, and I was in the World Trade Center hundreds of times well before 9/11. I think that part of the reason those attacks were so traumatic was that it wasn't just one plane crashing into one building. That was horrific enough. Then we get a second attack like the first. Then the Pentagon is hit too. Then we hear about the plane that crashed in Pennsylvania which apparently was headed for the White House. With each new attack, we could feel even worse, but I don't think it mattered to most Americans whether there were a few hundred people in the first tower attacked or a thousand. It certainly mattered to the families of the victims, but not to most Americans.
In short, I think the only way you can convey real loss to readers is to make an individual character important to the reader first. You can develop that emotional connection with a few characters, or a few groups of characters, but I don't think you can give a reader a strong emotional connection to forty individual characters unless you're going to write another War and Peace.
Even if you could establish such a connection, how many readers could stand to finish a book where you wipe them out, one by one? If you wipe them out in groups, maybe that's different, but we can only comprehend so much suffering at one time. Thus, I don't think the death of forty people can ever have forty times the impact that the death of one important character.