An article "A Hippocampal Cognitive Prosthesis: Multi-Input, Multi-Output Nonlinear Modeling and VLSI Implementation" by Theodore W. Berger, et al.
(https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3395724/) tells about a brain implant with 16 analog-to-digital converters and 32 channels of output.
I've been discussing it with Prozion (https://github.com/prozion) before and he has a very good question.
"How does the implant function at all with that amount of channels?"
The previous explanation from "Computational Cognitive Neuroscience" about memory de-indexing sheds some light on that: the article is about the same brain regions: CA1, CA3 and DG.