I have a blog hosted under blogspot currently PR4 My postings are just over 650+ My question is how many more postings do I need to move up to PR5?
The amount isn't the only factor. Quality is the other. It's better to have a backlink on a PR7 site than to have 10 on a PR1 site.
Number of links is only 1 factor, and most people are not too concerned about pr anyway, since in reality it has no major effect on serps.
If you just want to increase your PR, you should do some link building. PR is calculated based on the PR of your backlinks. The number of posts on your blog does not directly affect PR. If those posts are all interesting, they may develop natural links and indirectly increase your PR. Of course you should be more concerned with traffic and search engine rankings. PR alone won't help you much unless you are trying to sell some ads. Focus on building quality links with good anchor text and your search engine rankings should improve too.