Cannot view more than the last 9 posts on my user page.
You may have responded twice in the same thread, resulting in 9 threads visible rather than 10.
When you're logged in, you see the threads that contain your 10 most recent posts on your user page, plus all the threads you've bookmarked. When you're not logged in, you're seeing your userpage as others do - without the bookmarked threads, so with 20 recent posts on it.
I think it's actually 10, not 9, but yes, you're only able to see your 10 most recent posts on your My Chow page. You can neither expand nor reduce the number of your own recent posts you get to track in your "Recent Posts" list on that page. It's an automatic thing, a feature, a gift to you from the chowhound engineers.
However, if you post in a thread you want to follow, even after it becomes your 11th post and would therefore drop off your recent posts list on your My Chow Page, you can "bookmark" the thread. I bookmark lots of threads and haven't hit any kind of limit yet...