It's one of the few rules that they set out numerous times so I guess pretty serious. ALso they seem to be a suggestion that they will be using a bigger name cast (I assume they will broadcast the winning play) so it would cost more money to keep that up if yo had more than 12.
In terms of production costs,a minimum of locations/ sets would seem sensible and probably improve the chances of a submission actually reaching the stage.
"It's better to have loved and lost than to have married a writer - ask my wife."
