Set the IdleWinStationPoolCount to “suggested increments of 2″. This setting is under HKEY\LOCAL_MACHINE\ System\ CurrentControlSet\ Control\ Terminal Server.
Please refer to the following two articles in TechNet for more information about how Terminal Server creates and uses the idle port listeners and how Terminal Server manages ICA and RDP connections in general.
Q186572: Terminal Svr Walkthrough: Startup, Connection, and Application
Q238186: Simultaneous Terminal Server Client Logons May Not Connec