The original idea was to have a fleet of these, several, ready to go at any time so that if there were an emergency there were others that could go to their assistance.
That never materialized. They never had more than one ready to go at any one time as far as I know.
About 10 years into it(pre challenger) we were ramping up to do 2 a month. Obviously never happened.
Before Columbia there were times when a rescue shuttle could have been repurposed and prepped within a few weeks. After Columbia doing so was built into the planning, using the ISS as a lifeboat.
For the last Hubble Repair mission, where the shuttle (Atlantis) couldn’t make it to the ISS, a rescue shuttle (Endeavour) was actually prepped and ready to go, with the mission being assigned the designation STS-400.