Assume a generic room in a building is 20’ by 20’ by 10’ high, and has walls 1’ thick and a ceiling 2’ thick. That’s 2888 cubic feet of empty space in 4000 cubic feet, for a density 0.28 times the density of walls and ceiling, which, for the sake of argument, let’s say have 0.2 times the density of steel. Hence a generic building has a density about 5% that of steel. That is, you can build a volume of building 20 times, roughly, the volume of steel you have available.
The Earth’s crust is 5% iron, roughly. That is, 1/20 of the Earth’s crust can be turned into steel, roughly. Putting this figure together with that in the previous paragraph, we conclude that by using all of the steel in the Earth’s crust, you could build buildings with a total volume just about equal to the Earth’s crust. Let’s assume you don’t build on the oceans (70% of the land area). Then you could cover the land area with building to a height about 3 times the thickness of the crust, which is 3 miles, give or take.
Hence we conclude the ultimate iron resources of the Earth’s crust would suffice to cover the Earth in buildings 9 miles high.
On a more realistic level, we learn here that present iron ore reserves are estimated to be enough to produce over 230 billion tons of iron. At a density of 8 g/cm^3 that gives 2.9 trillion cubic meters of iron, or (from above) 58 cubic trillion meters of building. If the megabuildings are to be a mile high, there is enough iron known at present to build 14,000 square miles of them.
Either way, material resources have zip to do with whether you could build a `Blade Runner’ city.