Here's the results page for the most recent AQHA National Championship show.
http://www.aqha.com.au/shows_events/national_showQ10_results.asp
As you can see, there's not a huge number of competitors, from a very quick look the most I could find in one class was 18. Western riding just isn't that big over here, I'm in WA and you hardly ever hear of Western shows being advertised. Breed show will have some classes for QHs, but again there's not a huge amount of competition.
In contrast, one of the recent eventing competitions in WA had well over 200 entries - http://v3.ktrials.com.au/events/index.html/A472917E3/WoorolooHealthwaysGofor25CNC2223August2009