The number of DC vs Marvel comics is quite variable. It depends on various factors like publication schedules and crossovers. However, it's safe to say there are dozens, if not hundreds.
It's hard to give an exact number as new comics are constantly being produced. But there are a significant amount, probably in the hundreds or even thousands.
No, they haven't. Making a movie that combines characters from both Marvel and DC Comics is a complex process involving rights, creative decisions, and potential fan expectations. So far, such a project hasn't come to fruition.