RME uses FPGA's, which is why they're so expensive ( http://www.rme-audio.de/en_support_techinfo.php?page=content/support/en_support_techinfo_hdsp_totalmix_hardware ). It's several bazillion of these http://en.wikipedia.org/wiki/File:FPGA_cell_example.png that you can connect up into any electronic circuit you want.  I'd imagine they designed it so that each FPGA takes a certain number of channels and there's an addressing limit to how many FPGA's can be chained together in a multi-channel rig... thus the 64  channels of inputs even though each card has only 16 ins.

It seems like after so many years of production, now that they've stabilzed on a particular architecture, they could have come up with custom VLSI to replace the FPGA and save everybody a bunch of money; and allow their cards to be used by a wider number of musicians whose budgets send them to "prosumer" cards with chips like http://www.via.com.tw/en/products/audio/controllers/envy24/ ; these implement their 20-channel 36-bit wide digital mixer in VLSI, and derive significant economies of scale from the fact that they can sell one of their chips in every PC made.

I'm not complaining BTW, I'm just jealous of those that have RME cards. :-) For that price, I bet they don't stick any quasi-balanced outputs on their equipment and call them balanced like M-audio does for their Delta series .... ( which is actually "balanced" from an impedance and/or marketing perspective  http://en.wikipedia.org/wiki/Balanced_audio#Differential_signalling ). 

-- Niels