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//* Update number of posts per page on archive
function olivia_tag_posts_per_page( $query ) {
     // do not alter the query on wp-admin pages and only alter it if it's the main query
    if (!is_admin() && $query->is_main_query()){
        // alter the query for the home and category pages 
        if(is_tag()){
            $query->set('posts_per_page', 8);
        }
    }
}

add_action( 'pre_get_posts', 'olivia_tag_posts_per_page' );