An interesting question Paul. My wife just commented that if you thought of it earlier we could have got the answer for Australia tomorrow night.
I suppose it depends on how you define 'amateur astronomer'. Owning a scope is a good indication but I know someone who has had a lifelong interest in astronomy and is very knowledgeable but doesn't own a scope, and of course we all know of scopes owned by someone who clearly is no astronomer. There are a couple of things that it would be useful to know.
How many scopes are sold annually, for Oz and worldwide? How many have ever been sold (or built)? I know that is in no way a direct measure but it might help define the order of magnitude. When considering that number you need to remember
- there are more scopes sold than there are amateur astronomers
a scope can last for a long time
many astronomers have >1 scope
Another measure would be the number of members of astronomy clubs. Of course not every amateur is in a club and not all members of clubs are really astronomers, but my guess is that: number of club members > number of amateurs astronomers in Aust.