Yes the Pi's can handle up to 32 so i have heard it runs my 10 perfectly, but i would sack off the zoomhash image. It uses CPUminer and one session per gridseed unit, So if you have a problem with one you have to connect to each screen / cpu session at a time, it makes it very awkward plus it does not register the hash rate correctly, you can only measure it from the pool.

I built my image from scratch and put cgminer 3.7.2 with scripta on it. It works beautifully. So now all my gridseeds are monitored from one screen / cgminer session or from the scripta web front end. Plus if i want to change the pool remotely i just need to edit my daemon for cgminer and restart the miner and its changed. Where as the zoomhash image you have to run through that crap script and restart the pi its not ideal.