How many seasons are expected of Supernatural on the CW?

So far a season 4 and 5 have been booked but I am not sure about any others. Let's hope that there are many seasons to come on this brilliant show! Hope this helps

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Edit:

Despite Eric Kripke's original plan to limit "Supernatural" to five seasons, the show has been picked up for a sixth season as of Februrary 16, 2010. Jensen Ackles and Jared Padalecki are both on a six year contract for the show and will definitely be apart of season six.