I would say about 10000 to 15000. There have been almost 1000 registrations of MorphOS. If there were only 2000 users total, that would mean that half of all active amiga users are using MorphOS, which I doubt is the case. It's probably more like 10%, about the same for OS4, and 80% using Classic Amigas.
Edit. Forgot about UAE and Aros. Add another 1k for Aros users and probably several thousand using UAE but the vast majority just for playing old games which I don't think counts.
