Numbers-wise, 1-2 guitars per month, maximum.
Also, one I remember happening before - no re-entering guitars in more than one month (unless HEAVILY modified, and I don't think a refinish counts)
Also, if it's an award on a guitar-building forum, I don't think commissioned guitars built by someone else should be allowed. All those entering should have a hand in the construction process.
As for the pro/amateur debate, let both enter in the same competition, gives us all something to aim for.