I don't think there's a limit to the number of times you can apply to AdSense because I've heard of others who re-applied a lot of times before finally getting accepted.
While HubPages suggests that you have at least 10 hubs before applying for AdSense, nowadays, it seems that Google has become much stricter in accepting people into AdSense. There are a lot who kept getting disapproved for insufficient content. I myself applied for AdSense when I had 11 published hubs but I got disapproved for insufficient content. From reading around in the forums and hearing of other's experiences with AdSense, it seems that you'll need at least 25 or more hubs before you should apply for AdSense so you won't be rejected for insufficient content, which is why I haven't re-applied yet.
Also, before you apply to AdSense, be sure to read all of the terms of service, rules, policies, guidelines, etc so you don't end up inadvertently violating any of these rules. Google is really strict about that and you don't want to be rejected for breaking the rules or, worse, getting banned from AdSense.
It might also help to publish high quality content with lots of text, some pictures and even such things as polls or videos. You can check the learning center guide for a lot more information on these things. Here's the link: http://hubpages.com/learningcenter/contents