You can change this value by going to Settings -> Devices tab -> double-click the "Axon PBx" source and change the "Number of Channels for this Source" to whatever you like.
Note: Each instance of the source counts as an actual channel and may affect your VRS licensing arrangement, so please consider this when changing the setting.
Note: If you enable call recording in Axon then all audio in your phone calls must be routed through the Axon server. This may create delay issues in your conversations if there is a large distance between one (or both) of the call parties and the Axon computer.
For further information see here: http://www.nch.com.au/phone/voip/recording.html.