The amount of bandwidth used by RealPort is determined by how much data is going across the connection and the form that data is in. We do not have a simple rule of thumb for it.
The wan link speed variable in the driver can be used to limit the total serial bandwidth allowed to any given PortServer (note that this is serial bandwidth, not including tcp/ip overhead.)
The amount of data in any given packet is variable based on the application -- if it is a terminal, it is likely to mostly be one or two characters at a time coming towards the server, for example, while ftp/kermit downloads will have large data payloads per packet.
On newer PortServer units there is an option to configure for Throughput or latency. Latency option will increase amoutn of traffic on the network.
The amount of bandwidth realport uses in the idle state is small. It' is all on port 771 by default, so if you want to sniff your network, you will be able to see it and measure it. (The amount may vary based on network topology, and whether you have lan speed throttled down or not).
Because the amount of overhead/data is variable based on the data flow pattern, the only way to get a clear sense of how much bandwidth realport is consuming is to measure it on the network while all your applications are running. The bandwidth will be fairly consistent for a given pattern of usage, and should scale fairly linearly (as far as realport itself is concerned, i.e. discounting colisions, etc) if you add more units to the network.