Sticking with the original question, and speaking from my own statistics, I have found that 12 spoke wheels have performed well. It must be said, however, that any number of other factors could be involved.
As a rule, I have had less success with smaller wheels. I advise against the 6 spoke model for many reasons, primarily that a lot of the web 2.0 Google ranks are based on luck, and it makes sense to boost up the number of properties to give yourself the best chance.
My largest network is 28 properties and it has comprehensively nuked the search term I was going for. However, production and promotion took several days of hard work.
Yukinari - Do you have reason to believe that the web 2.0 services might penalise new properties from the same IP, perhaps by attatching a warning tag for google stating that they do not trust this property? It's an interesting idea as I have been noticing that the big services like wordpress and blogger have been generally under-performing for me lately. The last wordpress I made, however, did extremely well - might this contradict the theory?