This goes far beyond the scope of the demo lib code but to answer each of your questions:
Once an ANT slave channel pairs it saves the channel ID of the master channel it paired to. You must re-wild card the channel by setting the channel ID.
A USB stick only has up to 8 logical channels. However, if you did need to go beyond that number, you could implement ANT Scanning Mode which would allow you to "attach" or listen to about 40-50 HRM's simultaneously (it's limited by the UART on the USB stick).
I would strongly recommend reading relevant ANT documentation to learn about ANT and how to use it.
You may start from the
ANT Basics page and work your way through the steps. Afterwards I would recommend reading some app notes such as "
Device Pairing" and "
Continuous Scanning Mode".
Cheers