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This question is inspired by this one, specifically the part where it says,
According to various sources there were over 2,379,000 people on board the original Death Star when it exploded.
Were the Rebels aware of the number of people on the Death Star before the Battle of Yavin?
If the Rebels knew how many people were on the Death Star, it would probably have been from the stolen Death Star plans. I haven't been able to find anything definitive, in part because there are so many stories about how the Rebels acquired the Death Star plans. We might get a more definitive answer once Rogue One comes out.
That said, the Rebels did acquire the Death Star plans with enough detail to reveal the battle station's thermal exhaust port weakness (which was only two meters wide). It would be rather odd for these plans to include such details without providing at least a rough estimate of the required crew for the Death Star (e.g. for manning all the defenses). The Rebels could probably also estimate the number of personnel from these plans even if an exact figure wasn't given in the plans.
Additionally, recall that R2-D2 hacked into the Death Star's computers in order to shut down the trash compactor that was about to crush Luke, Leia, Han, and Chewbacca. R2 may have acquired figures for the number of personnel while hacking the computer.
The Rebels probably didn't know the exact number of people on the Death Star, but between the Death Star plans and R2's hacking they probably had a rough estimate.
New Canon:
Lost Stars
"Obviously, the Rebel Alliance was no better; it had blown up the Death Star with nearly two million people on board"
This does not indicate whether the Rebels knew that number or not, just that the number was exceptionally high. Once has to presume that given the size and scope of the installation, they had some idea of the resources needed to support it.
Unfortunately, the information in your question is no longer correct. The only canonical reference for the rebel estimates of Death Star casualties, as far as I know, is the new novel Star Wars: Bloodline.
“Don’t misunderstand me. Obviously the Rebellion was right to oppose Palpatine. Something had to be done. But if you ask me, that hardly condones the terrorist tactics of the Rebel Alliance.”
“Terrorist tactics?”
Casterfo stopped mid-pace; the two of them now stood in the heart of one of the long, dark tunnels, the wind rippling their robes and chilling Leia to the bone. He said, “Destroying the Death Stars, for a death toll of nearly one and a half million people, the vast majority of whom were low-level Imperial officers or even civilian workers?"
