Re: How many Shuttles could have been kept operative..
Those things were dangerous and inefficient.
Read Mike Mullane's "Riding Rockets" for a detailed description of just why the Shuttle was dangerous by design and how this was compounded by NASA HQ's array of PHBs and politically appointed numpties making the decisions.
Mike Mullane was a Shuttle-era astronaut and a very brave man who flew three missions despite knowing all the ways it could kill him and that its 'escape system', i.e. sliding down a pole wearing a parachute, was only usable before it had passed 50,000 ft on launch.