An educated guess (and it's *only* a guess) would be the following:
People using an original Amiga on a daily basis: 1500.
People with A1 waiting for OS4, or who will buy an A1: 1200.
MorphOS users, and people waiting for Pegasos II: 1200.
"Contributing"* UAE users: 1100.
Amithlon and other emulator users: 500.
Potential AROS users: 800.
Users who still hang around the Amiga community, but who have no Amiga hardware, except in a cupboard, and use Windows or Linux: 3000.
(* Contributing: who use these systems on a daily basis as replacement Amigas, not just to play favourite Amiga games now and again.)