because $rows always have the count of rows you could use:
$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)
$row_count--; // because first row start with 0 not 1 subtract one from row number (8-1 =7)
//now you could use $row_count to get last row (7)
echo 'testimonials_'.$row_count.'_testimonial'; //you need to replace echo by a variable and use that
Hint:
$rows--; is short form of $rows = $rows-1;
because $rows and better $row_count always have the correct row number you can add/remove rows without problems
