Planes Around the World Puzzle

The Puzzle:

Planes Around the World
You intend to fly non-stop around the world.

But a full tank only takes you halfway around the world.

However you can arrange many planes exactly like yours to assist with refueling.

You can only use the starting airport, and refueling can be done midair.

Ignoring refueling and turn-around times, what is the minimum number of planes you will need?
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