Before your loop create a counter, increment it and then break when it gets to 6
$count = 0;
while ( have_rows('t_block') ) : the_row();
$now = strtotime(date('Y-m-d', time()));
// I am assuming that your date is stored as 'Y-m-d'
// see strtotime php doc form more information
$end_time - strtotime(get_sub_field('ende'));
if ($now > $end_time) {
// not in the future, go to the next row
continue;
}
$count++;
if ($count > 5) {
// stop showing events
break;
}