That's another good reason to have extra channels. You could have different setups spread across the board, although that is probably something that someone with a bunch of keyboards would find convenient.
With the M520 and a 16 track. I have it set up like this;
Channels 1 through 16....tape in and outs via direct out
Channels 17 through 20....effects returns. So I have either 2 stereo returns or 4 mono
Usuallly I run stereo just for reverbs/delays.
All channels have direct inserts as well for individual effects on a specific channel.
In addition, I'm running protools through the console via the line inserts on each channel. So I can go back and forth from tape to protools at the flick of a switch.
To be honest , 20 channels along with a 8 buss and a separate monitor section has been plenty. No need for more channels
With a couple of patchbbays there's not much I can't do with 16 tracks.