Does anyone know what the maximum number of hard drives can be spanned together as a single drive letter (not NTFS mounted in a folder, and not hardware raid)

For example,

if you had 6x2TB hard drives = 12TB, can you span that together into a single drive letter, or is there a ceiling that gets hit at some point? Other than physical limitations of the case/mobo.

It would be in an x64 system (vista or more likely Win7) so there shouldn't be any 32 bit boundry to deal with.