ANSWERS: 3
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Four I believe.
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"Six shuttles have been built; the first orbiter, Enterprise, was not built for actual space flight, and was used only for testing purposes. Five space-worthy orbiters were built: Columbia, Challenger, Discovery, Atlantis, and Endeavour. Challenger disintegrated 73 seconds after launch in 1986, and Endeavour was built as a replacement. Columbia broke apart during re-entry in 2003." (http://en.wikipedia.org/wiki/Space_shuttle). With one (Enterprise) never having been space worthy and two more (Challenger and Columbia) lost in service, there are only three orbiters still in service as of the beginning of 2008. As for which has been the most productive, I don't know.
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Because of the Challengr and Columbia tragedies, only 4 shuttles still exist. I think the best mission was the Hubble repair mission
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