How many viruses are created annually?

I think that 400 viruses are created per year, or maybe it is per month. Or maybe the 400 is wrong altogether (which is why I am asking). If so many viruses are created, how can the anti-virus programs keep up with them all? There must be some that slip through. With so many hackers out there, I'm surprised there aren't more dangerous and undetectable viruses out there. Well, I'm definately not complaining, I don't want my computer being taken down by viruses. If it is 400 a month, though, then that would be its 4800 a year (wow, that's a lot!)