The magnitude of the populace of the galaxy is mind-boggling, so the concept of trillions of dead also is hard to grasp.
Innumerable planets, say a trillion people per planet.
Legions numbering from say 5k (Emperor's Children guesstimate) to Ultramarines 20k+ (they did spawn something like 20 chapters by themselves)...so an average of say 15,000 per Legion, and 18 or so Legions (270,000 Astartes on average...and these are the uber-elite, best of the best...and the majority of "recruits" never actually make it to become an astartes).
Add to that the thousands of IG Regiments (at approx 1k a piece), and say an average of 30% casualties...even 20%...it's easy to see how the numbers add up just among the military, not to mention the planets "cleansed". Frightning? Yeah.
I think Gav Thorpe captured the "Grim Darkness" of the future best in the first Last Chancers novel, where almost 2k prisoners were essentially liquidated for a squad to attempt a suicide mission. Life means little when you have trillions to throw away.
If say a dozen planets were effectively wiped clean, we would see several trillion dead,(easy) and that would still only be a small number in comparison to mankind in general across the galaxy. Large when you consider your place on a planet, and us as 21st Cent humans...yeah, the number is mind-boggling.
Then again, we ARE talking about math in the 40k(30k?) Universe...which has never really made any sense.
