It is difficult to say if 16GB will be enough for rendering heavy scenes since people have different opinions for “heavy scenes”, for some people 16GB might mean medium scene for others it might be a heavy scene.
I don’t think that there is a limitation for the number of the machines participating in DR but from practical stand point using too many machines is not very efficient since sometimes the first machines that receive the dr-job completes the entire image while the other ones still receiving the data.
Please provide more information about this issue - scene,screen-shots,versions etc.
The above is valid for V-Ray 2.x - with the release of version 3.x the License policy has changed.