Well if we have 98 billion people on earth - lets see how dense we get. This [site](http://www.zo.utexas.edu/courses/thoc/land.html) argues there's about 24,642,757 square miles of habitable land on Earth - this is discounting deserts and mountains, and of course oceans.
That puts about 6 people per acre in all the habitable spots on the globe.
To put that into perspective, Manhattan has about [27,000](https://www1.nyc.gov/site/planning/data-maps/nyc-population/...) residents per square mile - or 42 people per acre. There are other, denser cities out there as well. People-dense parts like this make other parts more people-sparse.
I'm sure by the time (if) we have 98B people - Manhattan will be much more people-dense, and many more cities will be similarly dense. So it may be that 50B people live in massive, dense cities, leaving the other 50B to be in more suburban-yet-still-dense zones.
However, this is the 'idea' case of all habitable land being used for habitation. There is also the need for farms and industrial land, and all manner of other places for human activity, and the vast things needed to support it. Ideas like vertical farming solve for some of these, but are not perfect.
98B might be possible if we did everything perfectly efficiently and optimized every single part of the planet leaving absolutely nothing allocated for the bare essentials to keep people living. The reality is we would never get anywhere close to that level of efficiency and would destroy the planet much sooner.
At Dutch population density (1,312 people per square mile[1]) that would be 32 trillion people so even 100 billion people should leave plenty of space.
That puts about 6 people per acre in all the habitable spots on the globe.
To put that into perspective, Manhattan has about [27,000](https://www1.nyc.gov/site/planning/data-maps/nyc-population/...) residents per square mile - or 42 people per acre. There are other, denser cities out there as well. People-dense parts like this make other parts more people-sparse.
I'm sure by the time (if) we have 98B people - Manhattan will be much more people-dense, and many more cities will be similarly dense. So it may be that 50B people live in massive, dense cities, leaving the other 50B to be in more suburban-yet-still-dense zones.
However, this is the 'idea' case of all habitable land being used for habitation. There is also the need for farms and industrial land, and all manner of other places for human activity, and the vast things needed to support it. Ideas like vertical farming solve for some of these, but are not perfect.