Known viruses means just that - that's the number of virus signatures in its database. Basically it means that it's scanned for all the viruses it knows about and hasn't found any - unsurprising as you're not running Windows.
This is as expected - as yet, viruses don't really exist on OS X. There's a couple of fairly lame trojans, but they're rare and not really something you're likely ever to come across unless you download a lot of iffy software from disreputable locations...