Limit for 8 Digi one EM with realport?

My confuguration: Win2000, Digi ONE EM. Realport ver 40002164-V or 4002164-W.
Is there any limitations for more than 8 Digi One EM?
The Open port with software (VB) for the 9th digi failed with “invalid port number”.
When the same port number exists, but only 8 Digi units - the open port success!