I would suggest you do not use a link wheel. It is pretty easy for Google to figure out what is happening and it may band your entire network. Build a link pyramid if you want to rank higher. Pyramid is much safer and yields much higher SEO results over time.
>>> I don't believe there is a limit. You can create a single link wheel using as many web2.0 sites. (b) which is better a open or close link wheel. >>> An open link wheel is better because these are more difficult to detect by search engines. (b) how to avoid SPAM while using a link wheel, is there any penguin update threat? >>> You can use spun articles. There is still a penguin update threat, so use spun articles instead. Hope that helps. Have a good day!