can you create sub-groups for example drums-guitars-voices to be processed separately and send them to the Main and keep the 12 stero mixes for example 12 aux?
Hi,
if you want full 12 mixes as AUX you can’t have Groups…
But… If you don’t need to apply processes on sub-groups you can use DCA to control level for lot of channels in same time 
sub-groups & aux are either/or 12 aux or 12 groups (or in between 1/11, 6/6, etc).
There isn’t 12 auxes AND 12 groups.
There are 3 Mtx
He asked to process groups of channels at once. Not control the volume of a group of channels at once
I don’t own a desk, but I found this in the manual:
In SETUP - MIXER CONFIG - BUS CONFIG you can set the number of groups and mixes. There are 12 stereo mixes and you can choose how many of them you want to use as subgroup instead of an aux mix. For example, you want to have 2 subgroups, than there are 10 aux mixes left. And you can select, if a mix or group is stereo or mono.
https://www.allen-heath.com/media/SQ_ReferenceGuide_v1_0_1.pdf