If you're talking Classic AmigaOS... Maybe 2 - 3000, about a tenth of what there was ten years ago. Maybe 750 - 1000 using OS4 in some capacity and I doubt very much higher numbers for MorphOS.
The C64 crowd by all accounts is a great deal bigger and always has been.
Just about, cause here in the Netherlands it's 100-200 at most, Amigas were very populair here, if you look at Germany it may be double and bit more that amount so at the most 300-500.
And then the rest of the world, 1000-1500.
Most Amiga's are laying in a dark scruffy attic or some wet basement, rotting away :confused: