1. Login to Feedburner account.
2. Click on Feed Title.
3. Click on 'Edit Feeds Details' link just below title of your feed.
4. In the Original Feed Address field, You can see your feed address.
http://MYBLOGNAME.blogspot.com/feeds/posts/default
Change this address to:
http://MYBLOGNAME.blogspot.com/feeds/posts/default?max-results=50
Instead of 'MYBLOGNAME' enter actual name of your blog.
Instead of the number '50' specify the number of posts ( 1 to 500 )you want to appear in your feed.
5. Click Save Feed Details.