Getting the comment number relative to all the post’s comments

Try the following custom Comment Walker class. The walker keep tracks for print index in a global variable named $current_comment_print_index; which is intialized in the paged_walk function.

You can print global variable $current_comment_print_index to show the current printed comment number.

<?php 
/*
Plugin Name: Comment Count Walker for WPSE 20527
Author: Hameedullah Khan
Author URI: http://hameedullah.com
Version: 0.1
 */

class CC_Custom_Walker_Comment extends Walker_Comment {

    function paged_walk( $elements, $max_depth, $page_num, $per_page ) {
        global $current_comment_print_index;
        $current_comment_print_index = 0;

        /* sanity check */
        if ( empty($elements) || $max_depth < -1 )
            return '';

        $args = array_slice( func_get_args(), 4 );
        $output="";

        $id_field = $this->db_fields['id'];
        $parent_field = $this->db_fields['parent'];

        $count = -1;
        if ( -1 == $max_depth )
            $total_top = count( $elements );
        if ( $page_num < 1 || $per_page < 0  ) {
            // No paging
            $paging = false;
            $start = 0;
            if ( -1 == $max_depth )
                $end = $total_top;
            $this->max_pages = 1;
        } else {
            $paging = true;
            $start = ( (int)$page_num - 1 ) * (int)$per_page;
            $end   = $start + $per_page;
            if ( -1 == $max_depth )
                $this->max_pages = ceil($total_top / $per_page);
        }

        // flat display
        if ( -1 == $max_depth ) {
            if ( !empty($args[0]['reverse_top_level']) ) {
                $elements = array_reverse( $elements );
                $oldstart = $start;
                $start = $total_top - $end;
                $end = $total_top - $oldstart;
            }

            if ( $paging ) {
                // HK: if paging enabled and its a flat display.
                // HK: mark the current print index from page number * comments per page
                $current_comment_print_index = ( (int) $page_num - 1 ) * $per_page;
            }

            $empty_array = array();
            foreach ( $elements as $e ) {
                $count++;
                if ( $count < $start )
                    continue;
                if ( $count >= $end )
                    break;
                $this->display_element( $e, $empty_array, 1, 0, $args, $output );
            }
            return $output;
        }

        /*
         * separate elements into two buckets: top level and children elements
         * children_elements is two dimensional array, eg.
         * children_elements[10][] contains all sub-elements whose parent is 10.
         */
        $top_level_elements = array();
        $children_elements  = array();
        foreach ( $elements as $e) {
            if ( 0 == $e->$parent_field )
                $top_level_elements[] = $e;
            else
                $children_elements[ $e->$parent_field ][] = $e;
        }

        $total_top = count( $top_level_elements );
        if ( $paging )
            $this->max_pages = ceil($total_top / $per_page);
        else
            $end = $total_top;

        if ( !empty($args[0]['reverse_top_level']) ) {
            $top_level_elements = array_reverse( $top_level_elements );
            $oldstart = $start;
            $start = $total_top - $end;
            $end = $total_top - $oldstart;
        }
        if ( !empty($args[0]['reverse_children']) ) {
            foreach ( $children_elements as $parent => $children )
                $children_elements[$parent] = array_reverse( $children );
        }

        foreach ( $top_level_elements as $e ) {
            $count++;

            // HK: current iteration index, will be added to global index
            // NOTE: will only be added to global index if already printed
            $iteration_comment_print_index = 1;
            // HK: count of current iteration children ( includes grand children too )
            $iteration_comment_print_index += $this->count_children( $e->comment_ID, $children_elements );

            //for the last page, need to unset earlier children in order to keep track of orphans
            if ( $end >= $total_top && $count < $start )
                    $this->unset_children( $e, $children_elements );

            if ( $count < $start ) {
                // HK: if we have already printed this top level comment
                // HK: then just add the count (including children) to global index and continue
                $current_comment_print_index += $iteration_comment_print_index;
                continue;
            }

            if ( $count >= $end )
                break;

            $this->display_element( $e, $children_elements, $max_depth, 0, $args, $output );
        }

        if ( $end >= $total_top && count( $children_elements ) > 0 ) {
            $empty_array = array();
            foreach ( $children_elements as $orphans )
                foreach( $orphans as $op )
                    $this->display_element( $op, $empty_array, 1, 0, $args, $output );
        }

        return $output;
    }

    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
                global $current_comment_print_index;

        if ( !$element )
            return;

                // increment for current comment we are printing
                $current_comment_print_index += 1;

        parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }

        function count_children( $comment_id, $children_elements ) {
            $children_count = 0;
            if ( isset( $children_elements[$comment_id] ) ) {
                $children_count = count( $children_elements[$comment_id] );
                foreach( $children_elements[$comment_id] as $child ) {
                    $children_count += $this->count_children( $child->comment_ID, $children_elements );
                }
            }
            return $children_count;
        }

}
?>