Use of MIDI-channels may be compared with the use of a selected frequency-band on radio or television. The receiver should listen on the same channel where the transmitter is sending information.
The MIDI-specification allows for 16 possible channels. Each channel than can have a single instrument pre-selected, and faders assigned to volume, panning etc.
MIDI-channel 10 is pre-assigned to percussion information. Whereas the other channels assume note-on and note-off events the drum-channel is special case, where each of the 127 notes is assigned a different percussion sound.
As long as you have 16 or less parts each part may be assigned its own channel, but when you need more than 16 parts, you have to make some compromises, as several parts have to share channels, and volume and panning-changes using the mixer will have side effects.