It all depends on your hardware configuration. As a rule of the thumb, you can have up to 250 concurrent users per server (not per CPU) as in most cases I/O is a limiting factor. Refer to the following links.

K2 blackpearl Performance Testing Results

Scalability, Clustering and Sizing

Ya to add to Sujeeth, the concurrent users will probably more be determined by IIS. Remember the user's interaction with K2 will be through ASP.net web pages, and probably not all of those pages will hit K2 directly all the time. So in total it will depend on what your entire solution looks like to determine the actual concurrent users.