I remember reading an article in the New York Times in 1999 in which they estimated the size of the Amiga community at 500,000. Obviously, there was no indication given of how they had arrived at this figure (someone's head seems like a probable source), but I recall being shocked at how low it was - in hindsight, I was no doubt being very naive. As was pointed out at start of thread, it all depends on how one defines an "Amiga User". However, considering that there were over 5 million Amigas sold in the UK alone, you would only need 0.2% of these to still use their Amigas to reach a figure of 10,000. I admit that this is probably way over the top but I reckon that there must surely be over 20,000 people worldwide who still use their Amigas on occasion - admittedley not as their primary computers - but, nevertheless, use.
Also, for this site to regularly attract 30 new members a week suggests that there may be a larger active Amiga community than one might at first think.