First, let's make sure we differentiate between sex (the form and function of the body) and gender (gender identity that's programmed into the brain).
We are all unique individuals, so you can say there are 7 billion different gender identities on Earth. Gender appears to exist on a spectrum; a sort of double Bell curve as seen here:
http://i147.photobucket.com/albums/r287/Igottherope/gendergraph.jpg
Most people identify with one of the two peaks on that graph (male or female), but not everyone. There is a small number of people who feel they are a mix of genders or don't feel like a gender at all.
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