Here we found out that, in some way, we can put an EMP device in a fake satellite on a low earth orbit.
A Web search revealed that a large device detonated at 400-500 km over Kansas would switch-off North and Central America.
But we need to switch off all the continents but Antarctica and Greenland. So, how many "EMP satellites" we need?
Using a (flat) world map and assuming a 2400 km (1500 miles) radius...
... we can see that we need about 15 warheads (let's say for now that the amount of damages inside the red circles is homogeneous)(oh snap, western Australia).
Assuming that this is a good strategy, how do we place fifteen EMP satellites in the right position if we have two or three launch facilities (anywhere you want) without arousing suspicion? (Let's say that the satellites are in a Faraday cage so they will not be affected by the explosions).