The number of posts displayed in the search results page follows the settings in WP Admin > Settings > Reading > “Blog pages show at most”.
The grid layout (2, 3, or 4 columns) is only available in Graphene Plus. If you’d like to modify the template yourself, the file is search.php. However, we don’t recommend doing this since any changes made to the theme’s files will be lost the next time the theme is updated.