To add to Rokket's answer, since the MR8 will only take two inputs at a time (i think that is true anyway), you will be limited to recording L and R outputs from the mixer at two tracks at a time.
If you are interested in recording 4 or more inputs at a time, I've heard interesting disussions about sub-mixes. I'm not 100% on this myself, so please forgive me if I am stating stuff you know well already. But the idea seems to be that if you have 2 inputs, you can either send a stero mix to the MR8 from the mixer (say you are recording drums, you have overheads, kick, snare etc all mixed and panned appropriately and there is a bit of everything in each channel) or if you have the busses, you can send a sub mix of say, kick and snare to one channel and a seperate mix with the overheads and whatever else to the other.
With the first option you have two stereo tracks "locked" with the mix from the mixer and any effects/eq added will hit all the instruments across the board. With the second option, you have 2 mono tracks that can be EQ'd or whatever and panned from there.
Daav