Yo Musicman! Don't you hate it when people blow off your honest questions? Generally the numbers indicate the number of tracks, and the number of busses. Therefore, a 12/4 mixer has 12 available tracks that can be configured in any combination to come out of 4 outputs, or "busses" Whether you need a lot of tracks or a lot of busses depends on what you are recording and what you need to do with the tracks. Once you have combined 2 or more tracks into a single buss, you better have gotten it right, because you can't separate them again. If you are recording through a mixer into a soundcard with only 2 inputs (stereo), then you would normally only be using 2 busses. However, additional tracks and busses can be used to send selected signals through effects chains, to create custom headphone mixes, for re-amping, lots of stuff. The more you learn about processing tracks, the more uses you'll find for extra tracks and busses.
Here's the problem- The more tracks you have, the lower the quality is per dollar. Sometimes, 2 damn good tracks are better than 64 that suck. Most good home studios have a few really badass channels that get used most of the time, and a bunch more that are "OK" for when you need a lot of tracks. Drums in particular suck up lots of tracks. Also, the more places you need to send tracks for processing, and the more mics you stick on a source, the more tracks you need. Personally, I am a minimalist, which means I don't use nearly as many tracks as a lot of guys, but I don't want any of them to be combined with any other track until I'm ready for mixdown. That way, if I don't like my mix, I get to just do it again until I'm happy with it. Lot's of engineers think nothing of using 64 tracks or more. I have no use for more than 18, but I want 18 competely separate tracks. I'm in the minority there, and most engineers will use lots more tracks than me.-Richie