How many Mac Viruses are there?

To answer your second question first, download ClamXAV (freeware).

To answer your first question: No actual viruses, although there are a couple of basic Trojans out in the wild - so long as you don't enter your admin password every time something asks for it (at least not without knowing that you're installing something reputable), you'll be fine.

As a fellow Australian ill give you a little advice. you dont need an AV scanner for Mac OS X. all they do are search for Windows viruses so you dont infect other PCs on you network.

if you ask me it should be up to the PCs to scan for viruses Mac users. also some AV scanners for Macs do more harm than good as they use way too many resources. if you really want an AV use ClamXav, this would be the only one i would recommend though.

tersono is right there have been a few trojans that can only install if you provide your admin password in an installer. but there a currently no Mac OS X viruses. my advice is to be safe and be your own AV scanner.