is it posible to separate "naipes"? sopranos from contaltos from tenor from baritones..?
Any way of getting 15 headphones?

A couple of years ago I recorded a cantata that was sung by a choir to backup tracks. I was using a Roland VS-880 so what I did was to record the backup tracks to the VS and use that for playback during the performance. I figured that way I'd have a direct recording of the backup instead of picking it up from the PA speakers. Then I set up my mics (4) so that I would pick up as little of the PA as possible. In addition to the fixed mics, I had an additional mic for soloists that was passed around from person to person as necessary.

The performance ran for four nights and I recorded every performance and did a comp of the takes to create the final CD.

An interesting side note is that one of the soloists had to miss a performance. When I set up for the recording that night, I copied his solo from the previous night to a spare track. I had the sound guy bring up that track in the mix at the point where his solo was and it sounded as though he were right there.

I've tried a bunch of these options and it really depended on the style of piece. Ideally, 15 headphones will work if you can drive them. If you don't do this regularly, I doubt that you would have 4 or 5 headphone amps. 2 solutions.
1. Put a set of monitors behind the singers - they don't need to hear clearly, they just need to stay with the track - right? EQ the hell out of the monitors to minimize mastering problems. Form the choir in an arc and keep the monitor as soft as possible and behind the singers- too much volume just makes singers sing sharp anyway - they need to hear themselves. Encourage them to overbalance the monitor. But...only if they can sing out in tune! (I've trained over 2000 singers in the last 13 years as a high school choral director - trust me on this one) - Problems are bleed and phasing issues but if you minimize volume it can work very well.
2. Put headphones on 1 or 2 people in each section. The rest will use the "leaders" as their monitor. Many decent choirs work this way intuitively anyway. A great choir doesnt' need "leaders" but most can function pretty well this way. The problem is staying with the click. I would do a few dry runs (keep it in record!) and identify problems spots. Then go for it and hope for the best. :)

I forgot to add:
For my ears, the most natural way to mic a choir is to fly 4 Crown PZM's sitting on a 1' x 1' sheet of wood or plexiglass. I've created a mounting system by attaching the bottom end of a mic clip to an L shaped bracket with a hole in it attached to the 1x1 square and then I can mount the mics on standard boom stands. In a room of your size, I'd put the singers at one end of the 20' and 2 mics at least 10, more like 15+ ft away. And using just 2 should minimize phase problems. But again, depending on the style of music, standing in a circle around an AKG414 also works great with a few leaders in headphones