How many shuttle flights per year without Challenger accident?

In retrospect we know that the O-ring issue that destroyed Challenger
was inevitable. It had almost happened in earlier flights and would
likely have happened on some winter day sooner or later. That said,
let's say it doesn't ever happen. Maybe because it's diagnosed, or
NASA and Thiokol listen to Boisjoly and require a certain outside
temperature for launches, or NASA just never schedules a launch on
that cold a day.

Without Challenger, shuttles would have continued to carry commercial
and DoD payloads. Discovery would be dedicated to SLC-6
launches. Additional American politicians are likely shuttle
passengers, as well as senior military leaders like Pete Aldridge and
more foreign dignitaries (Prince Andrew, perhaps?). The
Journalist-in-Space program would have joined Teacher-in-Space;
Artist-in-Space was likely next, and perhaps Writer-in-Space and even
Boy Scout-in-Space. In such a scenario it seems more likely than not
that Dennis Tito would have been able to persuade NASA to take him up,
and quite possibly for a price more akin to the $40,000 McDonnell
Douglas paid for each of Charles Walker's three flights rather than
the $20 million to Russia.

NASA flew nine shuttle flights in 1985, a rate it would never hit
again although it flew eight per year several times in the
1990s. Without losing Challenger, how many flights could NASA have
expected to have flown a year? I understand it was contemplating
perhaps two dozen a year with the four shuttles; was such a pace
feasible? If not that many, a dozen? 15? Would the agency order a
fifth shuttle with such demand?

<URL:> PERTH ----> *