Vicual C++ 6.0, maximum serial ports

I develop with Visual C 6.0 enterprise edition, under NT server 4.0. I have added a multi-serial port to extend system capacity (8-port digiboard). Although I can access all ports from a hyperterminal session, I am unable to access a serial port higher than COM9 from a Visual C developed application. Any attempt to access COM10 or higher using CreateFile will prompt a return code of 2 (file not found). Is it a limitation of Visual C, or did I miss something here?