How to Override “Blog pages show at most” in tag.php [duplicate]

what exact code should I insert on tag.php?

Nothing

To alter the main query, use the pre_get_posts action, which runs before the main query and before the template is loaded. If you modify a query in the template, you’re running an additional query, which is a waste of resources.

This would go in your theme’s functions.php, or a plugin.

function wpa_tag_posts_per_page( $query ) {
    if( ! is_admin()
        && $query->is_tag()
        && $query->is_main_query() ) {
            $query->set( 'posts_per_page', 10 );
    }
}
add_action( 'pre_get_posts', 'wpa_tag_posts_per_page' );