You are using the Recent Posts element which can only be used to display up to 4 items only. If you want to display 8 items, you will have to insert two recent posts element and on the second recent post element, please make use of the "Offset" option so that it will display the next 4 items.
And if you are using the recent element shortcode, you use two shortcodes like this:
[recent_posts count="4" offset="4"]
Hope this helps.