@Ursidae You can find out how many USB controllers you have by generating the HCL report. Sometimes the report shows controllers not connected to any ports (i.e. unusable), so you may need to do additional testing. In preferences of sys-usb, tab Devices, you can find out how many controllers are attached to the qube. You can split them into separate qubes and try to start/shutdown such qubes to see which devices from which actual ports appear/disappear from the system.
All USB devices on the same controller are not isolated from each other, and potentially one of them can compromise all others. The best security practice is to have one sys-usb per USB controller.