For example. If you are going to create 1 wordpress, 1 blogspot, 1 webs.com, 1 tumblr, etc. then you only need 1 IP.
If you are going to create 2+ wordpress, 2+ blogspot, 2+ webs.com and etc, then it depends on the site. The only thing you have to worry about if your web2.0 gets deleted, the others may get deleted also.
Using different or same IPs have no effect on search engine rankings.
Hope that answers your question. Your question is vague in terms of we don't know what your goal is or what you are trying to avoid/deter.