What is the requirement of Google AdSense for approval of blog? Is there is any need of back link before approval? How many post are sufficient for AdSense approval?

You don't need to have any backlink before you get approved for google AdSense. How ever, the following are few things to consider before applying for google Adsense.

you should publish ten articles

The articles should be informative, educative and valuable to your readers

Your website should be friendly and easy to navigate

You should also have the legal pages like privacy policy, disclaimer, and terms and condition pages.

Then your traffic source should be real visitors and not some sort of robbort