Hmm, how can this work?
With higher sample rates you have simply fewer channels due to port multiplexing.

The point in TotalMix FX's design seems to be, that everything is related to either physical channels or the number of channels that you transport via USB.

Otherwise it would also be easy to provide the virtual channels that I and other forum members proposed already,
to habe virtual channels for loopback recording, not to spoil real channels that you might need for other stuff.

The rules of thumb seem to be that
- Software playback channels simply match with the amount of HW outputs and
- The number of HW inputs and outputs shrinks as soon as channel multiplexing is in place.

Example: MADI with 64 input and output channels and M-1620 Pro.
At single speed you use 16 of 64 channels (in and out)
At double speed you use 32 of 64 channels (in and out)
At quad speed you use 16 of 64 channels (in and out).
By this you get a different channel count depening on the sample rate (64, 32, 16) and this also can't be changed.

Single speed           Double Speed        Quad Speed
Channel#                # (multiplexed)      # (multiplexed)
1                            1 (1                       1 (1   
2                                +2)                       +2
3                            2 (3                           +3
4                                +4)                        +4)
5                            3 (5                       2 (5
6                                +6)                        +6
7                            4 (7                            +7
8                                +8)                        +8)
...                           ...                          ...
64                          32                          16

^                           ^                           ^
|                             |                            |
TotalMix FX can show you only these input channels "real audio channel" (not the multiplexed)
And, as I mentioned above, you can't have more SW playbacks, because in the TM FX design # of SW playbacks and HW outputs need to match. Might have internal reasons ...

Ok, that would be great indeed; then my assumption was wrong.
Because then … you can simply keep your distribution of audio sources from the PC (could be plenty)
and distribute it to submixes (independent of the number of HW outputs/submixes).

Thanks, Matthias, for clarifying and later changing.