first you'll need some usb soundcards. these are pretty cheap, i think i paid $2.99 each and i got mine from ebay --> usb sound card
i might as well tell you what you will need.
1. a computer
2. usb sound cards
3. usb hub
4. audio recording software that supports multiple tracks (see below)
5. 3.5mm microphones or other audio out from devices
some software options are cool edit pro (used in this example), garage band, and i think audacity can do this too.
as for the input, you're receiving end is gonna be a 3.5mm jack, but there are all kinds of conversion cables and jacks to help you out. as soon as i got this working, i had grand plans of putting 6 or 8 of the usb soundcards and a hub inside a nice enclosure with some breakout cables which would extend the input jack to the exterior without having to do anything to the soundcards, and terminate it with the 1/4inch jack since i mostly use those for guitar and some of my mics. i never got around to it, but feel free to trick out your setup with a nice wooden box (i initially wanted to use a cigar box). don't forget to add a bunch of pretty lights. i still have a lot of ideas...
as far as i can tell, this system is completely modular, the number of tracks is limited only to the number of usb soundcards and usb ports you have. the most i've ever run at a time is six (i gave a couple away), but i don't see why it wouldn't work with more unless limited by your software or processing power.