URC8810: How many devices can the Portal Keys of the Smart Zapper operate simultaneously?

The Portal Keys are split up into four groups: "Power", "Channel Selection", "Volume control" and "Playback".
With the exception of the "Power" key, after having programmed the Smart Zapper using the Nevo App, the Zapper Portal Keys can be assigned by one device per group.
For example, volume control can be assigned to your amplifier, whilst channel switching is assigned to your set-top box.