FIVE. QUADRILLION.
MY HIGH SCHOOL LEVEL STATISTICS WILL NOT BE DENIED
I believe the actual population of the Imperium was five to fifteen quintillion.
Mmhmm. So you're telling me that each of the Imperium's one million planets has about 5-15 trillion people on it? On average, of course, I mean, the largest hive worlds are only around 2 trillion people, after all.
There's only actually about 32,380 hive worlds. 5th Ed Rulebook. Going by the wiki, which says between five and twenty hives per planet, and between 10 and 100 billion people per hive, the average might be around 500 billion people on a hive. Which is stupidly large amounts, mind you. I'm not entirely sure where the oxygen for that comes from. Maybe they import that along with crops from Agri Worlds. Of which you would need, well, several for each Hive World, but we already knew that.
Anyway. 500 billion times 32 thousand is about 16 quadrillion. That's just from Hive Worlds, but Death Worlds and Agri Worlds both have much less than a billion people each, Feral Worlds might have a billion or two, Civilized probably sit around 10 billion people per, and I have basically no idea how many servitors and tech priests inhabit forge worlds. Let's say 10 billion there, as well. If, say, four hundred thousand worlds are Civilized and/or Forge Worlds, then that's another 4 quadrillion there.
If we assume Pleasure Worlds, Cardinal Worlds, Shrine Worlds, etc. sit at an average of 5 billion, and that there's a hundred thousand, that's another 500 trillion. If there's two hundred thousand Feral World and equivalently populated planets, that's maybe 300 trillion more. If the rest of the million, now with just 270,000 planets left to look at, are mostly Agri-Worlds, Barren Worlds, and other things that might average around 500 million if we're very generous, that's about 135 trillion people. If we guess at another 65 trillion or so aboard ships and space stations, then that makes a nice clean 21 quadrillion.
Gotta say. 40k gets it's population number fluff to be quite reasonable, within context. Even if the actual equipment pieces are a bit ridiculous. (Land Raider Armor equivalent strength...and then plasma weapons are trillions of billions of millions of degrees or whatever bullshit >.>)