The current size of humanity can occupy 10,600 worlds, assuming the average is 5 billion per world (possibly more, as the average can be lower) Solarian population is quite literally going through a population crisis, considering the calculations show they would only have about 56 billion people compared to humanity potentially having a trillion. Solution could be that the Carcosans (placeholder name) have a significantly higher population, or the population growth is a lot higher than what I decided was reasonable for the calculations. (Favoring first option) About 30% of the Milky Way is already habitable, or is easily able to be terraformed by a Type 2 civilization. The best solution for the GCE population is to have it already have some sort of weak FTL travel before the main inciting incident for CS.