If I need to have multiple devices in one single app, would each Comm port be a separate device? I am asking about "Realms."

Multiple devices can share the same comport, which may be done for practical reasons like being a serial port connected to a single radio. VTScada can even share multiple protocols over the same comport (i.e. Modbus & Allen Bradley) saving the complexity of multi-port cards (though the protocols must be master slave and not confuse the remote devices).

With regards to Realms, they are more suited for isolation of tags and displays from user view and configuration where one application can be configured to be multi-tenant (like an apartment building) where users share infrastructure but maintain privacy.