Political compass

If that many people can fit in China and maintain a nation, I don't think the entire world is in danger of overpopulation.

Don't agree with any of the four. Overpopulation is why we need to reduce the number of kids we all have. No one needs to stop having kids and no one needs to die.

Yeah, solar system couldn't give a rats ass.

Well lets put it this way -- just because you CAN fit more people, doesn't mean you should.
My house could technically hold 30 more people and my local store could technically serve 2000 customers a day, but this is assuming that this is both ideal for anyone (which it isn't) and everything and every person is evenly distributed in an intelligent fashion so that you're not constantly struggling to cover the resource requirements (which it isn't).

You even say yourself 'some countries may be overcrowded' ... well acknowledging that space exists in the Sahara Desert doesn't change that.
Some areas are overly populated and others aren't... for a reason.
Again, just because you could theoretically distribute things to account for 50 billion people doesn't mean real world situations allow for it.

Sorry but I respectfully disagree with you. What do you think, exactly, would be the ideal human population?

The observation that most of the world's overcrowding is being done by non-whites is correct. Nearly half of the world's population lives in just sub-Saharan Africa and the southeast quadrant of Asia. Africa's overpopulation issues could be easily solved: stop sending foreign aid to the most resource-rich continent on the planet. Ultimately Africa's dysfunction boils down to three causes: low-IQ issues, agricultural issues, and disease issues - all three of which could be solved through genetic engineering, such as upcoming technologies like CRISPR/Cas9 and DNA printing.

"Well acknowledging that space exists in the Sahara Desert doesn't change that." Actually it is feasible to terraform the deserts or populate them using arcologies.
Then we also have polar, subterranean, and sea colonization - not just artificial floating islands, but also undersea habitats and bioforming - or editing our genes so humans can live in alternate environments, such as being able to breathe underwater and be more adapted to high pressure and low temperatures (and different dietary, hygiene, and immune system needs).

Then there are orbital habitats, such as O'Neill cylinders

Isaac Arthur goes well in depth on all of this on his YouTube channel:
https://www.youtube.com/watch?v=TqKQ94DtS54&vl=en
https://www.youtube.com/watch?v=XAJeYe-abUA&t=163s
https://www.youtube.com/watch?v=i6mRjsxQQJE
https://www.youtube.com/watch?v=b5NgMFdmQX0
https://www.youtube.com/watch?v=-ARhNOE4rEc
https://www.youtube.com/watch?v=GusIC3RMhbI
https://www.youtube.com/watch?v=1nZn3gMHPxg
https://www.youtube.com/watch?v=aQpYOVvU17Y