Source

File: wp-includes/class-wp-walker.php

public function get_number_of_root_elements( $elements ) {
		$num          = 0;
		$parent_field = $this->db_fields['parent'];

		foreach ( $elements as $e ) {
			if ( 0 == $e->$parent_field ) {
				$num++;
			}
		}
		return $num;
	}