Hiding all products except for one in wordpress admin panel

You have to use pre_get_posts hook to alter the WordPress main query.

$query->get() : https://developer.wordpress.org/reference/classes/wp_query/get/

is_admin() : https://codex.wordpress.org/Function_Reference/is_admin

pre_get_posts hook : https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts

// Handle the main query and set the post ID to 9
function wpse_288586_hide_products($query) {
    // We have to check if we are in admin and check if current query is the main query and check if you are looking for product post type
    if(is_admin() && $query->is_main_query() && $query->get('post_type') == "product") {
        $query->set('post__in', array(9));
    }
}
add_action('pre_get_posts', 'wpse_288586_hide_products');