The orange points are actual building data, the purple line is an approximation given by a simple exponential function h(n) = h₀ - h₁log(n) with h₀ = 452m and h₁ = 45m (or, if you prefer half-values h(n) = h₀ - h₂·²log(n) with h₂ = 31m.
What about the missing part, say n > 800 (or h > 150m)? Mindless extrapolation of the purple line towards the right produces a total number of buildings in the world of approx. 22000, which is clearly rediculous. If we reduce the average slope by a factor 4, we end up in the region of 10⁹, which should be about right.
For a similar graph for city skylines rather than individual buildings, see the World's Best Skylines - analysis.