Most of the many tens of thousands of sectors in the Milky War are defined by a Star's proximity to an astronomical marker of some sort, usually a nebula or star cluster. The Cairn Sector for example is defined as any star within 700 light-years of the Cairn Clouds, a large diffuse nebula that stretches several hundred light-years through space. Within this sector, there are 9 currently active civilizations. 5 Civilizations have advanced to space travel and are part of the Galactic council
The Vusinor are the most powerful civilization in the Cairn Sector and have seven stars they are constructing Dyson Swarms around, and within those systems, they have settled hundreds of planets and moons. Their population numbers almost 90 trillion, though they aren't even close to being the largest civilization in the galaxy. That honor goes to the Areakeyuk, a swarming hive civilization with a population over a trillion trillion times larger. The other four space faring civilizations are in an alliance with the Vusinor. They include the Cella and Seph Regalti, the Maorians, and the Yil.
This is not a typical sector in the galaxy. Most sectors don't have this many space civilizations living in close proximity. About half don't have a civilization at all. Not that it matters since virtually all travel is accomplished via warp gates, and around half a billion stars have had a functioning warp gate around them for millions of years, built by an earlier K2 civilization that has long gone extinct. Good thing those warp gates do exist, as FTL travel doesn't exist, and it can take upwards of 3 years to travel a single light-year with the absolute best technologies. Even the fastest civilizations will take hundreds of thousands of years to travel across the galaxy, which wouldn't be an issue if the universe was an empty place, but, it isn't.