ChocolateGork wrote:
Sure. But they have to be big assumptions. The creators of the 40k universe also had no sense of scale.

Deathwatch rpg Rites of Battle pg 98 wrote:War on All Fronts

The Achilus Crusade is a vast undertaking, one so prodigious as to hold the attentions of the High Lords of
Terra themselves. As such, Lord Tetrarchus feels the gaze of powers far higher even than him, and has more to
prove than any man save those gifted, some would say cursed, with the title ‘Warmaster’ can know. Hundreds
of worlds shall stand or fall; billions of faithful warriors shall live or die, all at the word of the master of the
Achilus Crusade.

So huge is the military and logistical effort required to maintain the crusade that no single administrative
body is in possession of the complete picture. Thousands of Imperial Guard regiments have been committed
to the conflict, yet scant few battle honours have been confirmed or allowed to be celebrated on the worlds of
their founding. Of the dozens of Cadian regiments committed to the crusade, only two have been decorated
for example. For every regiment whose efforts and sacrifices are officially recognised, hundreds more go
uncelebrated, the deaths of thousands of bold men and women unmarked and unlamented.

The numbers of Imperial Guard fed into this never-ending meat grinder can only be guessed at. At the crusade’s
outset, it is possible that as many as a billion soldiers marched to reconquer the Jericho Reach. As the crusade
found itself beset by far greater opposition than anticipated, this number steadily increased until an estimated
four billion troops were in action. The Officio Munitorum’s best estimates suggest that with the committing
of the theatre reserves formally held back by Lord Militant Achilus as many as six billion troops may be
operational. How many of these are serving in front line combat units and how many in support echelons and
service arms is a matter of heated, if ultimately futile debate amongst Officio Munitorum adepts.[u]