我认为，如果您当月没有8篇帖子，最好只进行第二次查询，例如：

//get current date
$date = getdate();

//setup args
$args = array(
    //basic stuff
    \'posts_per_page\' => -1,
    \'post_stutus\'    => \'publish\',
    //time parameters
    \'year\'           => $date[\'year\'],
    \'month_num\'      => $date[\'mon\']
);

//create WP_Query
$my_query = new WP_Query;

//execute first WP_Query
$posts = $my_query->query( $args );

//check first WP_Query, update if necessary
if( count( $posts ) < 8 ) {
    //update args
    unset( $args[\'year\'] );
    unset( $args[\'mon\'] );
    $args[\'posts_per_page\'] = 8;

    //do query
    $posts = $my_query->query( $args );
}

//do your loop and such