The answer is hardly at all.  We have many long hills with bog on top in my area of the North West, only a few of which have a scattering of turbines, erected by individual investors.  So we are no where near potential maximum density.

Some we may wish to leave clear for aesthetic and landmark reasons but much of the high land is not that spectacular, in my purely subjective judgment.

Speaking personally, I'd rather see an active productive landscape, like the old Dutch wind-powered landscape, than a pristine but unsustainable one.