Short answer: more than any of the worlds in the Solar System can accommodate easily.
Long answer:
When the Germans figured out that hyperspace travel could also be time travel, they set about rescuing their own ancient populations who had died in wars and plagues over the past 400 years. They would have gone further, but the sheer lengths of time in transit were a bit much. They had regen tech from the Mantid Empire so they could revive and heal the dead in one fell swoop.
Then they decided to go back in time, also 400 years, to create the colonies to put these people on. New worlds where the skills of the ancients were useful in farming and taming a new environment. The revived dead thought they were in Valhalla, at peace. Those populations thrived, some are now considered independent nations by the Germans in space. Billions on most of those worlds. They have been trading with their neighbors, both human and ET for centuries. While humans in Europe were still burning witches.
An entire string of colonies spread over a third of the Sagittarius Arm of the Milky Way. Overlapping with the Draco Empire. Thousands of worlds with over 2 billion each. And all of them have to come back to the Solar System. Somewhere.
These are educated people, with a culture. Used to having cyborg or robot slaves. Used to being master of their world, and an asset to the Draco. They are not going to be happy as refugees here.
You also have to think about the anti-German venom in the West on Earth. I know people who are calling them "proto-humans" and calling for their genocide. Who is going to accept literally trillions of Germans on Earth? With a culture that is hundreds of years in advance of ours?