The above joke title applies to Wing Chun especially as there is so much hair splitting. The answer is : 5. One to screw in the bulb - the other 4 to say they are doing it wrong !
I am prompted to say this because I saw on someone elses' blog they had a youtube.com video on their wooden dummy use. He had to disable comments as he was getting slated. Even if he was perfect in his sub-style of Wing Chun he would STILL get slated I bet (see above joke answer).
You don't get this in MMA, Thai boxing, Judo etc. These arts are very effective. They test their techniques full contact too remember - an acid test.