This is going back a few years, but we used on our systems, AD620 In-Amp (Instrumentation Op-Amp) front-end IC, and a PIC 16F series micro, some filtering in between, for 2 channels. Even today, I think one could get to 2 or 4 channels with wet saline solution, with a low BOM (Board-of-Materials) cost.

I used to repair EEG systems, and work as an EEG tech.