tried to this using
$rows = get_post_meta( 348, 'testimonials', false ); // get number of rows (8 but in a array)
$row_count = $rows[0]; //get row number out of array (8)
echo $row_count--; // because first row start with 0 not 1 subtract one from row number (8-1 =7)
//now you could use $row_count to get last row (7)
$lastrow = 'testimonials_'.$row_count.'_testimonial'; //you need to replace echo by a variable and use that
echo $lastrow;
What i get front end is
8testimonials_7_testimonial