system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] How big might a 'typical' regiment of the Astra Militarum be?

One from a small to average sized Hive world, say. Also, how often would such a planet form a regiment as part of their tithe?
gpt
There is sort of a disconnect between what happens, why, where and numbers in the case of the IG. For example a Hive world can have 100 billion people... meaning enough people would commit suicide a *day* to fill up entire regiments worth of soldiers however the issue then arises of singular regiments being raised up per year or even decade on these worlds. 

In the novel Baneblade for example the world is a hive world with a heavy link to the Adeptus Mechanicus and they only muster a regiment every few decades and sometimes not even that, they just reinforce the currently existing one that is travelling through to restock. 

In the Ciaphus Cain novels in the Leviathan (ice world) one, there are 3 regiments being raised from the ground up on an iceworld that has as you would expect, a small population.

Then we get to Krieg which after its civil war sent out 10 regiments immediately following the conclusion and is constantly pumping them out despite the you know... radiation poisoning and lack of everything in general. 

The Vostroyans constantly reinforce their regiments already in the field, sending out thousands at a time. 

Basically nothing makes sense and its all up to the discretion of the Planetary governor and how much he/she can get away with at the time from the looks of it because there appears to be no unified answer.

It must be noted that Agricultural Worlds, Space Marine claimed worlds and Mechanicus worlds all send smaller if any in terms of tithe (human or material) due to pre-existing treaties or lack of demand i.e. produce is more important than people in these situations or they just dont want to piss off the Space Marines. Feral/Death worlds again make little sense as some export singular regiments (Catachans) while others according to records export dozens at a time but only every few decades.