"Darkhive" works on the assumption that all of the people in the setting are mixed blooded descendants of an original "Astral Cruiser" adrift for millennia in the furthest reaches of hyperspace, it works off the standard minimum numbers for a "generation ship". New blood only comes from other ships caught in it's mystical "gravity wake", ships that have crashed into it's outer hull. Then the setting increases those numbers by a reasonable magnitude so that a range of unique cultures might develop.
"Walkabout" works on the assumption that Earth has been hit by a metaphysical apocalypse, where the spirit guardians of the planet have risen up against the atrocities casued by man and have taken matters into their own hands after millennia of sleeping. 99.9% of the population is wiped out (90% through trigger nuclear defenses and war between nations when the apocalypse hits, 90% of the remainder in rioting and lawlessness after the infrastructure of society decays, then a final 90% are unable to survive the nuclear/metaphysical winter, before the game begins in a new spring). In a global population of 8 billion, this drops to about 8 million...in a continental population of about 25 million, Australia drops to 25 thousand. The city of Sydney would drop from 5 million to 5 thousand. This might be a bit too much of a drop to be sustainable in the long run. The numbers have never been hard and fast.
When looking at survivor numbers in Walkabout, we're looking at the number of people with good stories to tell, and people who have something special about them to help survuve the worst of times. As a society, we've become attracted to post apocalyptic fiction over the past decade because it is a scenario of equalisation, money counts for nothing when the banks and credit industry is destroyed, fame counts for nothing when the communication systems are gone. If you can do something practical, you're chances of survival are higher. It's a chance to shake off "big government" and "big corporations" to prove that we can do it on our own. The problem is that most people can't. I've always run "Walkabout" in my head with a basic survival formula based on the size of towns where people currently live. In bigger towns and cities, people are more interconnected and rely more heavily on the infrastructure around them; people in smaller towns are more independent; and people who live alone in the wilderness don't rely much on that global infrastructure at all. There may be more survivors in larger towns, but as a percentage of original population, there will be more survivors in smaller towns. This works in Australia because most cities and towns have vast tracts of open land between them, and each village/town/city can be modified as a discreet unit.
Here I apply a formula where "survivors = (cube root of original inhabitants) times 10". So, in a city of 1 million people, we end up with 1000 survivors. A larger city of 3.375 million would end up with 1500 survivors, a small town of 4000 people would end up with 200 survivors, and a tiny outback homestead of 27 people might actually grow to 30 after a few survivors managed to survive the trek to somewhere safe. I've used nice numbers that find easy cubed roots for these examples, but you get the idea. It's not a perfect system, but it's a good starting point.
This brings me to heroes, protagonists, allies, and the other characters in a setting that are comparable to the player characters. We play these games and tell these stories for the purposes of escapism, we like to portray unique and individual snowflakes, set apart from the wider world through our special powers, abilities and narratives. Where we draw the balance says a lot about the stories we'll tell.
If everyone exists at the same power level as our avatars within the world, then their really isn't anything particularly special about our characters. That's not a bad thing, it just means we'll probably be telling grittier stories about the common person in this setting. Some of the characters encountered will be more powerful than the protagonists we are portraying, and at these times our story might veer toward that of the underdog. More commonly though, our roleplaying sessions work on the assumption that the characters are more powerful, and set apart from the wider populace in some way.
In the Classic World of Darkness, there was a ratio given for vampires in the world. A population in any urban environment should not exceed 1 in 100,000 mortals, otherwise the masquerade might be more likely to be breached. This worked well in a city like Sydney with 4-5 million people because it gave 40-50 vampires (certainly enough for interesting politics). In smaller towns that might limit vampires to less than 3, not enough to really form a good party of player characters, and certainly not enough to bring antagonists of equal power into the mix. Still, it's a useful ratio to think about.
A game like L5R works on the conceit that the characters are members of high ranking families within a pseudo-Japanese empire, with retainers on command and numerous peasant around them as they perform their feats of heroism and honour. Large families might have tens of thousands of members, small families might have only a few hundred, and then there are the thousands of peasants who have no formal "family" but are basically treated as cattle and slaves to keep the empire running. Here it might be better to look at a formula like the "Walkabout survivor". Let's say there is always a single leader of a family, they have an inner circle of heroes, courtiers and generals, the there is an outer circle of lesser heroes (among whom are the player characters), then a wider group of valued retainers, and everyone else linked to the family are unnamed peasants. The characters exist in the middle with respect to power level, but toward the apex of the pyramid with respect to numbers. I can't remember if there is a minimum "family size" in L5R, but I do remember that every Great Clan has 3 main families and a scattering of smaller associated families (lesser clans typically have only a single family, and assorted retainers without family name).
Let's say a family has a minimum of 500 members, and may have up to 20,000.
There would be 1 family head (or daimyo).
I'd run with an inner circle of "fourth root times two" (10 for the smallest families, and up to 22 for the largest)
Then an outer circle of "cubed root times five" (35 for the smallest families, and up to 135 for the largest)
A number of valued retainers equal to "square root times ten" (200 for the smallest, up to 1400 for the largest)
Then everyone else is an unnamed peasant (255 for the smallest, up to about 18500 for the largest)
If we look at the player characters being in that "outer circle" group, and their valued retainers in the next group down (where inner circle members have twice the share of retainers as the PCs) we see that members of small families might have about 5 retainers at most, while members if large families might have up to 10. Bigger families mean more power, but far more politics to get even further ahead.
The stories change based on the numbers.
I'm thinking of basing settlements within the "Darkhive" on this set of numbers I've shown for my L5R games (using levels of "Settlement Leader", "Counsellors and Elders", "Heroes and Captains", "Contacts, Allies and Militia", "Survivors and Scavengers"), and similar for the mutants in "Other Strangeness" (using levels of "Force of Nature", "Powerful Mutant", "Starting Mutant", "Special Human", "Regular Human").
As always, things are subject to change.