On macOS Big Sur 11.6.8, I've created an aggregate device using the Presonus StudioLive 64S and a couple of Roland synths as ONE interface which I've called StudioLive 64S AG. Happily, all of these inputs show up in StudioOne with the OUTPUT sends going from Studio One to the StudioLive 64S inputs. They arrive at the desk without issue, I can even use the desk's internal FX post Studio One! Powerful stuff!
Not sure if there are any upper limits but I'm using 94 audio inputs and 72 outputs without issue. Incidentally, the Roland audio devices are the Juno-X and Jupiter-X which each have 6 Stereo outputs via their USB port. I've used all 6 stereo outputs (12 mono) as inputs on the Aggregate device in Studio One without issue. See attached.
One word of warning though, if you want to use the Jupiter-X or Juno-X as aggregate devices then you need to open their driver setups to reduce the audio buffer size or it will crash DAWs like Studio One. Let me know if you have any questions. Thanks.