Tim,
So, it might be 200, maybe 250-300, assuming some courses can do double duty.
About 50 for the PGA Tour, maybe 40 for LPGA, and keep the rotation of 10-15 each for US Open and PGA. Most regional events are contested at less than the 7,500 yards for the PGA Tour, being set up to "protect the bottom half of the field". (Last time I checked, a few years ago, the average or median PGA Tour course length was 7,209, oddly a distance most top club players seem to like. They are a bit shorter than the top PGA Tour players and still like to have fun, hit greens, and score well.)
However, I agree it will take some doing for high end clubs to take out the back tees and it may be enough to make them 15 x 15 feet squares with lots of natives between them and the fw. The real problem in this proposal is that it would probably take an outside board (like the EPA) to force this on golf, unless we can somehow self-govern looking to the greater good rather than our own egos about rankings, difficulty, etc.