-Open up the Mix console.
-Go to the Channel visibility pane on the left hand side
-Shift click to select a lot of channels.
-drag the slider down to see more channels.
-Shift click on some more channels (trying to increase the amount of channels selected)
-Doesn’t work. GUI takes you back to the first channels you selected and wont let you select any more.
I did find a workaround however. Shift+arrow down will allow you to select more channels.