Currently, you can have a maximum of 32 computers in a cluster.
Having more computers means more network traffic, so there might be a small performance hit in that regard.
Having the clients perform all the rendering is perfectly fine. Simply add the following code to your script to prevent the master from rendering:
Code:
viz.cluster.pushMask(viz.MASTER)
viz.MainWindow.visible(0)
viz.cluster.popMask()