I have played several games. Getting the Eyes of the Universe Wonder (it is worth Rushing it, since it gives nice adjantcey bonuses to neighboring research districts) and the 2nd Subspace tech (Subspace Scanning (1st) to Subspace Streaming (2nd)) make it much easier to find the other Sectors and Races. Then you can use a Surveyor type ship with infinite range to find where all the Races are. Zoom way out to the point where you can still just see the locations of all of the Hyperspace Drive Home Worlds (from Eyes of the Universe) and then do a right click Near each Race HW and you will traced out a path to reach it. Right clicking ON the HW icon will not work, you are clicking on a location inside a star. This path will show that path in a dotted/dashed form going through all of the Sectors and Subspace Streams to get to it. If you do this with only the 1st Subspace Tech (Subspace scanning) and NOT the 2nd one, you can get paths ONLY to Races whose Sectors are linked by ONE Subspace Stream to your Home Sector. With the 2nd Subspace Tech you can get paths traced through many Subspace Streams and Sectors.
In my most recent game (to 2311), Large Map, 16 Races, gifted, there were at least 17 Sectors, with several (at least 4) small Sectors with NO Race in them. Many of the Races were the only Race in their Sector. My Home Sector had 3 Races, and 1 or 2 Sectors had 2 Races.
In my game before that, Gigantic Map, 16 Races, normal difficulty, there were at least 36 Sectors, with my Home Sector having 4 Races and again most of the other Races were alone in their Sector. That game I played out to 2313 with Huge Hulls Tech, before starting a new game. Some of the Sectors were large and others smaller, sometimes not much more than one or Stars.
I had several other games as well before those two games.
If you do not trade away any of the Techs leading to Subspace Scanning, I have NEVER seen another Alien Race find their own way to Research it. The same is almost true for many other Techs, e.g. those leading to bigger Hull sizes, it will take a VERY long time for those Races to find their way to them. If they are isolated in their starting Sector, they will have a difficult time getting Universal Translator Tech and therefore everything that comes from that as well.
I do not know how true this will be for Games at even harder difficulties than Gifted Races.
added: I started a new Medium size Game Map with 16 Genius Races. I have not played long enough to know much about it's Sector layout, but from the Eyes of the Universe, I can tell 2 Sectors with 4 Races (one of them mine, along with the Altarians, the Plants, and the Spiders), 1 Sector with 3 Races, and 3 Sectors with 2 Races. I know nothing about Sectors with no Races, or how they all connect together. I also made it a game with NO Tech Trading, which changes the flow and feel of the game a lot.