Username    Post
demo         2
admin        1

Username    Post
demo         3 <-- Because the user has asked the Question / "thread" and has replied twice
admin        1

public function number_of_replies($user_id, $thread_id) {
    $this->db->select('*');
    $this->db->from('reply');
    $this->db->join('thread', 'thread.user_id = reply.user_id', 'left');
    $this->db->where('reply.user_id', $user_id);
    $query = $this->db->get();

    if ($query->num_rows() > 0) {
        return $query->num_rows();
    } else {
        return 0;
    }
}

<?php

class Who_replied extends MX_Controller {

    public function __construct() {
        parent::__construct();
    }

    public function index($thread_id = '') {
        $data['users'] = array();

        $data['thread_id'] = $thread_id;

        $results = $this->get_users_who_replied($thread_id);

        if (isset($results)) {

            foreach ($results as $result) {
                $data['users'][] = array(
                    'user_id' => $result['user_id'],
                    'username' => $result['username'],
                    'total' => $this->number_of_replies($result['user_id'], $thread_id),
                );
            }

        }

        $data['total_posts'] = '';

        return $this->load->view('default/template/forum/categories/who_replied_view', $data);

    }

    public function get_users_who_replied($thread_id) {
        $this->db->select('user.username, user.user_id, reply.thread_id');
        $this->db->distinct();
        $this->db->from('reply');
        $this->db->join('user', 'user.user_id = reply.user_id', 'left');
        $this->db->join('thread', 'thread.user_id = user.user_id', 'left');
        $this->db->where('reply.thread_id', $thread_id);
        $this->db->order_by('thread.user_id', 'desc');
        $query = $this->db->get();

        if ($query->num_rows() > 0) {

            return $query->result_array();

        }
    }

    public function number_of_replies($user_id, $thread_id) {
        $this->db->select('*');
        $this->db->from('reply');
        $this->db->join('thread', 'thread.user_id = reply.user_id', 'left');
        $this->db->where('reply.user_id', $user_id);
        $query = $this->db->get();

        if ($query->num_rows() > 0) {
            return $query->num_rows();
        } else {
            return 0;
        }
    }
}