function like_content($content) {
if(is_single()){
$google_ads='
здесь ваш рекламный код
';
$button= '<div style="float:right;padding:8px 0px 8px 8px;">'.$google_ads.'</div>';
$poses = mb_split(' ', $content);
$counts=sizeof($poses);
#----------------------------------------
$pickme = $poses[$counts-55];   #<---------- ставим нужное число!
#----------------------------------------
#$content = substr_replace($content,  "<div>" . $button . "</div>", $pickme, 0);
$content = str_replace($pickme, $pickme.$button, $content);
//reset it
$lastpos = -1;
return $content;
}else{
return $content;
}
}
add_filter( 'the_content', 'like_content' );

$content = str_replace($pickme, $pickme.$button, $content);