There are hundreds of billions of stars within the Milky Way. That much has been speculated to be true.
I have heard that the average Hive World contains between 100 and 500 billion citizens, and also that Hive Worlds make up 14% of the Imperium's worlds.
So does anyone have an estimate as to the total population? An Imperial-wide census would be absurd, of course, but has anyone heard anything ever?