if you're recording lots of tracks and/or using lots of effects.
snare top
snare btm
high hat top
high hat btm
tom 1
tom 2
tom 3
over head Left
over head right
kick drum out
kick drum in
room mic
there's 12 right there for an average drum setup. Using more drums, you'll need more mics and more channels.
Then you go onto the other instruments
gtr1 mic 1
gtr1 mic 2
gtr1 DI
gtr2 mic 1
gtr2 mic 2
gtr2 DI
bass DI
bass amp
solo guitar
keyboard
lead vocals
bg vox1
bg vox2
And this can be all for a typical rock band. If you get into an orchestra, jazz band, or larger more complex band of some sort, you'll get more channels
Then you can go onto using effects for all these channels. And if you're running a console as your sole mixer, you'll probably be returning these to open audio tracks.
Or if you're using the console for a larger project like say, a movie...each individual sound can get it's own channel.
Of course, this is more common in the professional world. For home projects, you'll rarely see large consoles...especially if you plan to mix all inside the box.