As someone who knows a bit about the demographic shift and how population sizes change both during and after industrialization, the idea of galactic empires with hundreds of billions or even trillions of humans never made any sense to me. I guess overpopulation was a serious public concern in the 1960s, but statisticans had already figuted out that the massive global population boom of the 20th century would not continue indefinitely into infinity decades earlier. Industrializing societies grow rapidly. Industrialized ones don’t. From all the data we have, if they don’t get large scale immigration, they actually begin to shrink, and quite dramatically. Eatimates are that the human population of Earth will peak at 11 billion at some point in the mid 21st century and then most likely tank dramatically over the next 100 years unless extensive government programs are set up to encourage people to have at least 2 children on average. There just isn’t going to be any people to settle hundreds or even thousands of planets without radical breeding programs. Programs that would cost a lot of money without providing any benefits, unless humanity were in some kind of cosmic war of attrition. I guess it would work for Warhammer 40k.
A very simple solution to having a species colonize numerous planets while being demographically plausible is to simply have colonies with smaller population sizes. We have started using the word billion so commonly in our modern language that it can be easy to forget that a billion is a thousand million. Even if just a single percent of a species of 10 billion individuals lives on other planets than the homeworld, that’s still 100 million people. That’s a scale like having 10 countries like Austria or the Netherlands. You could even have 20 New Zealands or 65 Hawaiis. Have a whole colony exist as a single primary settlement and you can have a city of a million people. That’s not as big as New York, Chicago, or Tokyo, but it is a seriously big city that could even have a few modest skyscrapers if it wants to feel a bit fancy.
If you have some 10 alien species with around 10 billion people each, even when 99% of them live on the 10 homeworlds, you can still have several hundred colonies with poplations in the millions. And many many more with populations in the hundreds or tens of thousands. A billion colonists can populate thousands of colony worlds if you spread them out a little.
Now a new question I’ve been pondering this week is how long the travel times between inhabited worlds could be without regular contact between them appearing implausible. I quite like the idea of moving them really quite far apart so that in situations where the players find themselves tied up in some local crisis, it can take weeks before outside forces can arrive to interfere. I think that makes local politics much more meaningful and gives the players much more time to solve dangerous situations themselves before the cavalry arrives.
The idea of transplanting small populations to really far away places and letting them fend for themselves, and people actually signing up for it because it sounds like a good life is not a new one. It actually happened several times in our fairly recent history. The main examples that always come to my mind are Honolulu in Hawaii, Perth in Western Australia, and New Zealand. A hundred years ago, the Hawaiian islands had a population of 250,000 people, 84% of which had been recent imigrants, mostly from Japan and the US mainland. At that time, Perth had a similar population, and New Zealand a population of just over 1 million. Since then, Hawaii has grown to 1.5 million, Perth to 2 million, and New Zealand to 5 million. If you look at a globe with Hawaii in the center, it almost looks like it’s the only significant piece of dry land on the whole planet. From Perth, it’s thousands of kilometers of open ocean to the West and South, and thousands of empty desert to the North and East. It is easy to imagine them sitting on a different planet entirely.
Today, getting to these places is really quite easy. 4 hours on a plane to reach major global population centers really is nothing. But it is not that long ago that people accepted a very different reality as simply being part of life. A convoy shipping soldiers from Austrlia to Egypt in 1914 took seven weeks to cross the Indian Ocean. Reinforcements for Pearl Harbor in 1944 took 12 days to travel from Los Angeles to Hawaii. Hawaii in the 1940s was not some primitive agrarian society that was agriculturally self-sufficient to see to all its simple needs. It was a fairly modern world with refridgerators, gas stations and radio music, really not that different from ours. If they could live in modern comfort this far removed from any other civilization, then so could space colonists with all kinds of fancy fabrication technologies.
When considering travel times between planets, 10 days in hyperspace to reach the next nearby colony or 50 days to travel to the homeworld are absolutely justifiable. Of course they would be remote, but that wouldn’t make them cut off from the culture and economy of galactic society. A small colony of a few hundred thousand people a month of travel removed from their homeworld can still have all the comforts of civilization, with access to advanced medicines for even uncommon ailments, and getting imports of music, movies, and games from all across the galaxy.
10 days of hyperspace travel between systems on average feels like a good guideline for my own setting. It would be plausible for the kind of societies I am imagining. Also, building on my earlier calculations for the economy of interstellar shipping fees, having something shipped to the next planet would cost about 10€ per kg, assuming a small space freighter can be bought for 5 million € and you can take 5% profit after operating costs. You wouldn’t want to pay that much for shipping for grain or concrete, but for importing electronics or medicine that a small colony can’t produce locally, that’s actually pretty cheap.
Nothing of this is in any way a solid model (because the capabilities of space ships are completely made up), but you can poke a setting like this quite a lot before it starts showing serious holes.