I read that on the Auria DAW forum. The post was by a developer in answer to someone who asked why they saw all their interface inputs but only two outputs.
I don’t have a multichannel interface so I can’t test it. On the Auria website there is a list of interfaces that have been tested. The answer wasn’t “get one that works”.
I've never tried it, but I have USB channel restrictions in my head. USB2 is restricted to 54 I/O total, iOS brings that down to 30. Since you can only have 24 inputs, that leaves (at least) 6 outputs.
These are channel number restrictions for RME devices, based on hardware restrictions. If you need more channels, you need to use Firewire, Thunderbolt, AVB, or Dante.
I'm familiar with Core audio on MacOS, which doesn't have restrictions and I believe Core audio on iOS is largely the same, but the hardware is the resctriction, as fi USB2 is less performant.
Maybe an old post on the Auria forum?
I've read on the audiobus forum that their devs promised to have multi output available "soon". Whatever that may mean. And that was a post from november `18.