With the QU series the mixer surface 16, 24, 32 is the channel limit even with using a stage box, you can mix and match inputs from the stage box and the mixer but the total input is limited by the mixer.
The exception is the QU SB and QU Pac, those can be expanded to a total of 32 channels when using a stage box.
That all said with using a stage box you can get a few extra channels by assigning a stage box inputs to the stereo channels on a QU mixer.