I've had a good look at the documentation and I'm no closer to knowing the answer. Is there a limit to the number of outputs that can be defined for an Azure Stream Analytics job?
There was a limit of 5 outputs, which is currently removed. Classic portal still shows a warning message when user tries to define more than 5 outputs. It is all but a warning though. New portal (Ibiza) do not have this warning.
Also current limitations can be found on this page.