If I remember rightly, I understand that some 5 million Amigas had been sold up until 1990 or 1991, so the total of 7 or 8 million that has been mentioned several times could be quite accurate.
Australia was a surprisingly good market for the Amiga. I remember hearing figures of 700,000 to 1 million being quoted. Personally, I would go for the 700,000. It's worth noting that even now, there are still three active Amiga user groups here in Melbourne.
I can also recall one of the British Amiga magazines actually had an article on this matter back in the early '90s. Unfortunately, I can't remember which magazine.