Is your blog with WordPress.com? Just wondering because when I click on the link you provided, the blog posts seem to come from a self-hosted blog using WordPress.org software. If you need help, you’re better off asking over at http://wordpress.org/support as they’d be able to help you more.
I think this is simple enough to do, though; from what I remember of my .org install, what you want to do is go to Dashboard > Options > Reading and there should be an option for syndication feeds. I think changing the number there should do what you want.