The only truly exclusive Mosquito races are the state titles (Victoria and South Australia) and the Australian Titles, but at some regattas like the Sauna Sail in VIC we get our own division.
It's fairly common for any class that can get 10 boats to a regatta to be given their own division, and in Vic the Mosquito has been achieving that more often than most classes. That's not to say there's more Mozzies than anything else, but many other classes don't travel much these days.