Maximum distance a plane can fly is 1/2.
Plane A will go around the world.

1. A ans B start the journey.
2. At distance 1/6, A takes 1/6 worth of fuel from B, B stll has 1/2 - 1/6 - 1/6 = 1/6 worth of fuel to go back.
3. A has an extra 1/6 worth of fuel that enables it to fly to 1/2 + 1/6 = 2/3 distance.
4. Plane C travels from another side of globe for 1/2 - 1/6 = 1/3 distance and makes a rendezvous with A. C gives A a half of fuels in tank which is 1/12. Both A and C will have 1/4 left before out of fuel.
5. Another 2 planes D and E wait at 1/4 and fuel 1/4 worth of fuel to A and C, then they all have enough fuel to make it.

So we have plane A B C D E, five of them.