At any one time, NASA had, at most, four space shuttle orbiters: Columbia, Challenger, Discovery, and Atlantis. Endeavor came online after the loss of Challenger, and there were no replacements for Columbia. They only generally prepared one orbiter at a time, although I do recall some early attempts at back-to-back launches and discussion of having a backup available for flights post-Columbia where two orbiters would conceivably be prepped at the same time.
Given that at most two orbiters would be being prepped in the Vehicle Assembly Building, where were the other two (or three) during that time? Were they tucked into a parking spot in a corner of the VAB? Is there a separate hangar(s) that they would be parked in? A nice tie-down on the ramp in between a couple of C172s? Given how delicate the orbiters really were, I’d expect somewhere climate controlled and obviously pretty secure, but I don’t remember ever seeing anything about “parked” orbiters?