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Settings>About Phone>Android Version.
From what I can tell in the
manual, I don't think you can modify the number of homescreens using the stock launcher. You could consider installing a 3rd party launcher like
Nova, which does allow you to modify the number of homescreens, as well as other aspects of the launcher interface. Nova requires Android 4.0 and up, so if you have a version that's lower,
GO Launcher might work better.