If you're from India, you have to wait 6 months before applying for AdSense. But I think there were others who have been approved even before 6 months. Perhaps you can try once you already have a lot of hubs.
There's no clear number of hubs that you should have before applying for AdSense. HubPages suggests that you have at least 10 hubs before applying but, nowadays, Google seems much stricter in approving people to AdSense. A lot have been rejected for insufficient content and those that have been recently approved said that they had at least 25 or more hubs before being approved.
Also, make sure to read all of Google's terms of service, rules, policies, guidelines, etc to make sure that you're not violating anything as that can be a cause for your AdSense application to be disapproved.
To increase your hub scores, it would help if you participate more in the community such as by posting in the forums, asking and answering questions, publishing a hub, etc.
It's up to you how often you want to post hubs. If you finish writing a hub, then you can choose to publish it already if you want. I think I read somewhere that posting too many hubs all at once can affect your scores and you should avoid doing that.
Check out the HubPages FAQ and learning center guide as both contain a lot of very useful and detailed information such as regarding the hub scores and even AdSense. Here are the links: https://hubpages.com/faq/ and http://hubpages.com/learningcenter/contents .
Welcome to HubPages. Hope this helped.