Not that it's likely much use to others but my set up is a bit different to the usual sound card.
I have
a Yamaha DM1000 digital mixer (48 channels on 3 layers) which I use for input. I've equipped the mixer with 2
MY16AT expansion cards which give me a total of 32 ADAT inputs and outputs. These are connected to an M-Audio Profire Lightbridge interface then into the computer via Firewire.
The interfacing is completely duplex but, with only 48 channels on the mixer, obviously I can't have all 32x32 going at once. However, the whole thing can be configured with presets so, at the push of a button, I can jump between set ups. Typically, I have 16 inputs to the computer (and it's rare to even use that many at the same time) and 32 outputs back to the mixer.
My actual mixing is "in the box" but bringing all recorded tracks back into the mixer allows me to set up customised monitoring for the headphones of the musicians--I have 8 auxes available, all switchable pre/post. All routing is very flexible--for example, I can feed my headphones at the desk with anything from an individual channel to a whole rough mix, to listening to any of the headphone mixes.
There's also a compressor-limiter on every channel--I don't track using compression but I do keep a hard limiter on in a "just in case" mode so if one transient would clip, an otherwise good take is protected. Hopefully the gain structure is such that the limiter is never hit but it's comforting to know it's there if somebody decided to yell. Finally, there are 4 inbuilt effects units--again, I track dry but these can be routed to the monitoring if somebody asks.
Frankly, this system architecture owes more to the live sound work I do than to my home studio needs. I rarely record even 16 tracks at a time, much less 32. And with mixing ITB, having all tracks separately into the mixer is a bit of a luxury.
But it can be fun!