For our Mikes...
1-Audience
1-Primary vocal
2-backup vocals
If you have 3 mikes , you'd be OK with the Traveler, 4 preamps.
( wich is a wonderfull unit BTW , I should have got that one instead of the 828 .... )
You can Apply EQ and comp on all the Inputs AND all the Outputs. All your mixes including if you make an FOH mix have a Full parametric 6 band + LPF/HPF.
You can have a limiter for all the stereo IEM mixes wich is a must IMHO. (all I/O have comp)
You can also have the reverb send from the Inputs and from the Bus send. Usefull when you want to apply the same Room to a complete mix and not just some instruments.
The reverb return is either at the bus return or at the output.
All your Mixes (bus) can be assigned to whatever output on the unit.
Also worth mentioning when plugged to a CPU , there is :
An FFT Analysis
Oscilloscope
X/Y Plot
Phase analysis
Tuner
Those function uses the CPU power , it's not internal.
Also one disadvantage of these units, the processing power is dependent of the sampling rate.
Meaning if you run @ 44.1Khz you'll have the FX at all the I/O (28 inputs , 30 output for the 828 )
but if you run @ 96Khz you'll have half these.