I ran into a problem when I wanted to reduce the number of posts in the default "recent posts" block. This block is created by the blog module and the number of 10 posts is, unfortunately, hard-coded. You could add your blog feed to the aggregator and using its block. You can then specify the number of posts in either the aggregator properties or in the block created by it.
That works for a single blog but for multiple blogs you would need multiple blocks and that's not what I wanted.
I saw a nice snippet on the Drupal site dealing with this.