You can try to get more blog posts on your main page with this jQuery mod:
But, I do NOT recommend it! Here's why. I believe Ning probably wants to cap the number of blog posts because it is a strain on their database to send out too many at a time. So I would ask at help.ning.com before you try to increase the number of blogposts you display. Use this at your own risk! It may be a TOS violation, I can't say for sure. If it is, Ning may delete your network!