Another excellent point...the more busts of Ed McMahon show up the less-likely I'd think people will accept it as a result of natural forces.
So does quantity of evidence need to be added to our list? I'd think it would eventually play a part in the meta-analysis following initial evidence collecting. Someone has to ask if we've collected enough rocks to start classifying them. Which forces the question of how many is enough to draw a conclusion?
I think there are of course many cases that are a gray area when it comes to judging "design or natural." Remember the "face" on Mars? For a while there it could have been interpreted either way. The thing we must recognize is that these things are all interpretations. But there is a threshold in which concluding something was formed by natural causes becomes absurd. And like wise the same can be said for concluding something to be designed. If I were flying over a small island and saw "natural" drift wood formed on the beach to say, "Marooned...send help!" It would be absurd to conclude they just randomly floated in to that position. Likewise if I saw a cloud floating by on a lazy summer's day that kind of resembled the cartoon character "Sponge Bob Square Pants," it would be equally absurd for me to think it was "designed" by an intelligent source. In both cases I am relying on my own knowledge of human activity. I am not aware of any giant cartoon cloud making machines anywhere made by humans, but I am aware that only humans can form coherent sentences out of drift wood. So in one sense it is a "quantity" of evidence because of our knowledge and understanding about human activity so far. But that doesn't mean we need to see several busts of the same thing to accept it is designed. I would say that all we need is common sense.
I think at some point we have to recognize that this ain't "rocket surgery."
I would rather inspire one, than impress a thousand.