Blogger sends latest 25 posts in your blog feed. So your Feedburner feed also contains the latest 25 blog posts. If you wish to include more or less posts in it :

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.