Yikes! These rwenty great voices are on a stage, in a room, in a semi-circle.. on platforms of different levels?
I think Bill's got a good starting point, and my only addition would be to listen carefully to them in the room, find an area where you can hear everything well, then see if that same position, in front or above them looking down a bit, works as bill suggested .. what I'm trying to say, is don't just dump them dead center and live with that .. I'd mic them from a couple of different locations, carefully mark down your settings, and distances, make a drawing, and take a photo .. then listen back .. if you have that option during a rehearsal.. go for it, you'll learn a bunch and have some pluasable options.
I did a session where I had:
Timpanis, trombones, saxes, clarinets, tuba, french horns, flutes, and oboe all at one sitting. I don't think it could have been worse, I'd never done anything like that.
I chose to place them in a circle, the timpani outside and back a bit from the circle. They were all sitting down, so I placed two AKG-451's in an X/Y overhead, pointing downward, and then placed a Lawson L-47 in the middle about seat height in omni. I got lucky, and you could very clearly hear everyone with no overpowering. I did mess with the overhead exact placement a bit and same with the low placed omni so they balanced... really quite lucky. The point being I couldn't hear them well as I walked around until I was inside the circle.
edited part: mixdown:
oops .. multed the 2 tracks of akg's so that two tracks were panned hard left and hard right, and the other two were in that 10:00 / 2:00 area, with the omni tcak dead center, and multed as well in the mix to two more tracks to add just a breath of bottom, panned however it sounded well .. yep a mono signal going three ways .. center and L/R panned to taste.