The maximum number of turbines that can be placed is twelve, which provide a total of 14 units of energy.
There are also several arrangements with just one fewer turbine (eleven). All of these simple patterns, unfortunately, produce less energy than the original arrangement. This is because the number of turbines in windier (blue) areas has yet to exceed the four in the original arrangement.
We can take one of these arrangements (bottom left, above) of eleven turbines, however, and shift three more turbines to windier areas, producing 14.5 units of energy.
Would removing one more turbine improve the energy output further still? It would if we can move all three remaining turbines to windier areas, because the one unit of energy lost would be offset by the increase of 1.5 units of energy from the three turbines in windier areas. This indeed can be accomplished to produce a maximum of 15 units of energy.