How to reduce channel count dynamically in mc ?

Hello,

I would like to change dynamically the number of channels in mc. i.e. reducing the number of channels starting from 32 and changing dynamically between 0 and 32 channels, throwing away the excluded last channels and not mixing the 32 original channels into a smaller number.
Is there a simple way to do this ?

I have tried mixdown, but it doesn't seem to do it (activechans mixes). I have tried to unpack and repack, but pack chans number doesn't have a chans argument... ???

Thanks for your help.