It seem to me that there isn't any physical limit. Probably is a practical limit though. There are a few four nozzle extruders coming to the market now. Not sure how they work but might give you some ideas. The Kraken (http://e3d-online.com/The-Kraken) is a bowden fed four nozzle hot end. I guess you could put two of them on a reprap fairly easily to get eight different colors.