From Fender’s Facebook page:
When Jonny Greenwood joined the London Contemporary Orchestra on July 28, 2014, the Radiohead guitarist used 12 Fender ‘65 Twin Reverb amps, sending live guitar loops to individual amps for a true surround-sound experience! #thisisthesound http://bit.ly/1vnAkwc
As you might expect, the setup was used for his performance of Steve Reich’s Electric Counterpoint. Likely, each of the pre-recorded electric-guitar backing tracks went to an individual ‘65 Twin Reverb Reissue. However, since there are only seven electric-guitar and two bass-guitar backing tracks (though the bass may not have gone through the Twins), it is unknown what the extra Twin Reverbs were used for. Note that these were not used for the lead/solo guitar track, which Jonny played live through either his Silverface Fender Super Reverb or his Vox AC30. I suspect, the former, since he generally seems to use the Super Reverb for Electric Counterpoint, and an AC30 for his own solo compositions like Loop and Self-Portrait with Seven Fingers.
The most likely option in my mind is that all twelve amps were never used at once. In this scenario, the amps were adjusted to different settings, and so different tracks were sent through different amps at different points during the performance. For example,
I. Fast - Twin No. 3
II. Slow - Twin No. 11
III. Fast - Twin No. 3
However, it is also possible that each Twin simply reproduced all of the tracks. This makes for interesting surround-sound, but it makes the use of so many amps a lot less necessary. It is also possible that a couple of the Twins were reproducing the more important tracks multiple times.
Regardless of the exact use, this is surely what Jonny was referring to when, prior to the performance, he tweeted about a “new take on Reich electric counterpoint (technical side willing).”