The answer of the Vienna based 4youreye, Lichtapette and Kraftwerk.at is 15 with a total output of 42 displays.
Created for Club Vanilli in Linz, Austria, the trio assembled 3 Mac Pros and synchronized them using MIDI. Each machine is running Modu8 of course. If you can read German the full article can be read here.
The case study is also available in English: http://www.matrox.com/clubvanilli
Thanks for the tip