http://blogs.technet.com/b/markrussinovich/archive/2009/09/29/3283844.aspx is a good read on this topic. Actually, the number I quoted was for something else entirely - number of threads. The Windows limit on files is MUCH higher. It is a limit per instance of the OS, so with MPI across multiple computers, each computer has its own limit. This assumes, of course, that the files are being opened locally and not using a network file system.
As best as I can tell, it would be very difficult to reach this limit, though there are other resources, such as virtual memory allocated to keep track of each open file, etc. Also, if you open up thousands or millions of files, the Fortran I/O system may have reduced performance.
