Barry, you can open as many RXR's as your CPU and GPU in your computer can handle, but they have to be within the SDRC's/RXR's bandwidth.
i.e. My SDR-iq has a max bandwidth of 190 kHz, so I could open several RXR's within that 190kHz of spectrum. For reference, my AirspyHF+ is 768 kHz, and my SDRPlay is 2Mhz(?).

You can also open numerous 'stances' of SDRC and connect to multiple RXR's for listening to numerous frequencies in the various bands.

What exactly are you attempting to do? i.e. listen/record  several HF stations at the same time that are tens of MHz apart?

Kriss KA1GJU