I think the big question here is not how many viruses are created but how many computers are affected. There are I bet thousands of viruses created every month, because any body with basic computer programming knowledge (which is a lot of people by the way) can create a virus. Since virus is something that is destructive it is easy to create one. I mean I bet there are a lot of people who can create distruvtive macro which could delete all the system files, which is also considered as a viurs... But the important thing is how many of those viruses can the 'evil people' spread around, and often it is just a few of those (when I mean few, I mean a few of those million viruses) that enter the general person's computer.

Like William said, rules can always be broken (easily, for some pieces of software), and that goes for every piece of software. So it's impossible to know how many viruses are created, but one thing might be certain, the more people learn programming, the viruses might significantly increase.