But those 81 cities take up a tiny portion of global land area.
A quick Excel import of the table from Wikipedia suggests that the "urban area" for those 81 cities is 190 K KM(2).
Total land mass on early 148.94 M KM(2).
So the world land could hold ~63,400 such cities.
It would likely take many years to build a city like Guadalajara (#81 on the wikipedia list).