Colors the capt. is right that actually happened to me I had a Yamaha 12 channel 2 of the channels were stereo it wasn't really enough for our 3 peice band. I can tell you that the Allen & Heath is the best mixer out there for the money it has the best pre-amps hands down. I got the Mixwiz 16-2 for $863.00 from NSL the light was an extra $100.00 I used a label gun you know the kind that prints those sticky labels and made 3 labels for each channel one for the trim at the top of each channel one for the fader at the bottom of each channel and one for the corresponding imput on the snake. On the mixer I put the vocals on channels 1 thru 4 with extra channels in 5 and 6 just in case someone sets in, or a fader goes out, acoustic guitar on 7, keyboards 8, lead guitar on 9,base guitar on 10, and drums 11 thru 16. If you have a soundman it is always best IMHO to lay the drums out on faders as he see's them from the audience perspective example, kick drum 11, floor tom is on the left in ch.12, low wing tom 13, high tom 14, snare 15, high hat 16 it makes it easier for him to find the correct fader when making adjustments or in case of feedback. I hope this helps.
I say go for as many channels as you can get, if you can find a 24 channel you wont regret it. We use a 16 and we are basically 3 piece and we use all of the channels up. We rack mounted ours but we do have an old Mackie 24 channel in the trailer we could pull out if need be.
Good Luck and Keep On Rockin