AntiVir - which, out of all AVs, has the worst score on the Virus Bulletin website - supposedly has a database of over 80,000 viruses. F-Prot for DOS - perhaps the best free AV out there - supposedly scans for between 70,000 and 80,000 viruses. I'm pretty sure I've heard of commercial AVs that (supposedly) scan for even more. (Panda, IIRC, supposedly scans for over 100,000.) 20,000 different nasties is a lot, but is it enough?