Their is still enough room for everybody, Even if the population doubles, tripples, the major citties will build bigger housing more everything, We wont have yards or things like that anymore but, people who live in Shanghigh usualy dont even have kitchens just a closet with a bed and a toilet. In Tokyo they are building the first building city, I think in the next 100 years or so these building cities will be everywhere. If the Space aliens or the Religons dont kill us then Sky City in Japan is the future. It is only logical before we can colinize space we will have to live here.

Sky City 1000 - Wikipedia, the free encyclopedia

it says a possible future but they have already laid out foundation and started building, it is very impresive, everything from water purification, housing and food distibution, jobs, shops, are their. No need for a car if you work, and live in the sky city.