More content is usually better. Although, there isn't a specific amount of content that AdSense requires, it appears that they are not as concerned with number of Hubs as total content. So a bunch of short Hubs might not work as well as fewer longer and more in-depth Hubs.
I think a month and 15-20 featured Hubs of good length could be a good starting point.
Please keep in mind that since we are not privy to the entire Google AdSense application process, all of our suggestions are guidelines and should not be taken as Google gospel!