To keep an easy overview, not all channels are included in the default mixer layout. You may add any idle channel or channel pair via the Add Channel drop down menu. Channels that are already displayed in the mixer are “grayed out”, channels that you can add are “solid”.
Please note: Max I/O mixer channels are 24 in / 8 out. You might have to remove unused channels in order to add other channels. Channels that can be removed have a small crossed square in the upper right corner. De-click this field to remove the channel from the mixer. Channel 12 is used by default as talkback - remove talkback channel in order to add channel pair 11 + 12 via the Add Channel drop down menu.