Yes, I am afraid the maximum MIDI channels for AU is 16 at the moment.
Recently I managed to increase the VST3 channel count to 128, which is supported by several DAWs. For AU there are some non-standard tricks to increase the channel count (very complicated, using a dedicated MIDI CC and scripting, I think), but I have not spent time with it yet.
I hope someone will correct me if I am wrong, but I think an AU MIDI FX plug-in is not able to output more than one channel (maybe through IAC MIDI ports?). The track will receive the MIDI directly from the plug-in.
I recommend using the RC AU plug-in which routes MIDI messages to a virtual MIDI output port called “RapidComposer Out”. You can receive MIDI messages from this port in your instrument tracks. MIDI routing is unnecessarily complicated when using AU.
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