The basic principle is that you're "mixing" x number of inputs into a pair of outputs. Professional studio mixers tend to add a plethora of additional features.
Perhaps sharing my commonly-used setup with the MultiMixer will give you an idea:
Channel 1 1/4" mono (panned to Left) - XV-5080 L
Channel 2 1/4" mono (panned to Right) - XV-5080 R
Channel 3 RCA mono (panned to Left) - JD-990 L
Channel 4 RCA mono (panned to Right) - JD-990 R
Channel 5 RCA mono (panned to Left) - LAPC-I L
Channel 6 RCA mono (panned to Right) - LAPC-I R
Channels 7/8 1/8" stereo - SC-55mkII L/R
Channels 9/10 1/8" stereo - MU-100 L/R
Master Outs L/R RCA - DS-50a monitors
Line Out 1/8" stereo - connected only when recording (see the "Composition suggestions" thread for examples...)
Headphone Out 1/8" stereo - not connected
Anyone currently swapping audio cables between devices needs a mixer.
Anyone currently swapping devices in and out of their mixer needs one with more channels.
