When I read the first post I though the OP wanted to buid a "standard mixer".
Now I understand that his needs are simpler.
He has 20 line level, low impedance outputs.
If he does not need individual volume controls, a passive mixer (it does not needs to be balanced) will be enough.
Using 10K resistors everywhere, it will have an attenuation of 20 times, eg - 26 dB.
I think he will need some monitoring amplifier, that can easily supply the gain needed to compensate the attenuation.
The amplifier will have a volume control, to adjust monitoring level.
If he wants to go into experimenting, all this can be built with a "chip amplifier" (LM3886 or equivalent, boards can be found on ebay) summing the channels' resistor on the volume pot.