You can open in your program as many serial ports as are installed in the computer.
Probably the best option do perform monitoring tasks is to install a callback for every port opened and manage the traffic in the callback.
The throughput of the application depends heavily on the amount of traffic monitored and on wether you have only to "hear" and save on disk / display messages or you need to decode and react someway to them: I have tried to build a "simulator" for some devices of our production that was intended to decode the requests and give correct answers but failed to stay in time limits with more than a few devices monitored.
Hope this helps
Roberto
