You can find the code here:
wp-content/themes/symbiostock/inc/classes/widgets.php
You would need to change line 800 from:
'posts_per_page' => 12, // return 5 posts
to:
'posts_per_page' => 6, // return 5 posts
But please note that any updates that you make to the parent Symbiostock theme will overwrite this.
The best thing would be to put this in your child theme, but I don't know how to do that for a class that is below the main theme directory (wp-content/themes/symbiostock). If you figure that one out, then please let me know.