I have this loco and run fairly often. I have never heard these (or any of the Century Club offerings) were limited, rare, or otherwise special other then how they were sold (direct). I understand the way it was marketed might be viewed as "limited" but in reality, these was never a limit, implied or printed.
As far a actual numbers, we, the general hobbyist will never know, but I am sure a few people and one in particular who views this board has the answer. However, I don't think he needs or feels he has any reason to provide this information.
As far as this specific loco, the sound package, although old and dated by todays standards, is still impressive especially the whistle. The "chuf" is very good, deep sounding, but only two per revolution.
In my case, I have been spoiled by cruise control "Odyssey" in Lionel speak, and honestly, this is the main reason I find I am running these engines less and less. That might be the reason you see more of these on the market... that as well as people dying and the widows selling stuff just to get rid of it.
Enjoy your engine, it is a pretty good runner for a non-cruise control locomotive.