For a double window you can easily get by with using just two curtain panels on the double window.
But for a much better look it's best to use three to six curtain panels to use 3 sections of curtains as in one curtain on each side and one curtain in the middle.
So for the 3 curtain look you'll need from three to six of the curtain panels but for the other look two curtain panels will be enough for a double window.