This article uses modulation tricks to try to get many inputs all on one audio channel.
However, USB sound cards, available for around a dollar[1] have a nice high input impedance and can directly be connected to EEG probes for direct sampling of microvolt level signals. No circuitry needed!
You can easily connect hundreds of them to the same system and get hundreds of input channels with great electrical isolation. The way USB works, they also have perfectly synchronised clocking too!