Total number of active sessions

You might need to write a custom session handler to insert the session data into a database.  I have just moved our site from using file based sessions to DB based for pretty much the same reason as yours.  Writing the session handler only took a couple of hours :)
Try this:
<?php

/* Create new object of class */


$sql_table = "Session_Table";

$lifetime = get_cfg_var("session.gc_maxlifetime");

function open($session_path, $session_name) {
      
      mssql_pconnect("HOSTNAME", "USERNAME", "PASSWORD")
      or die("Can't connect to mssql server! ");
      
      mssql_select_db("[My-Database]")
      or die("Can't select mssql sessions database");
      return (true);
}

function close() {
      
      return true;
      
}

/* Read session data from database */
function read($ses_id) {
      global $sql_table;
      $session_sql = "SELECT ses_value FROM " . $sql_table
      . " WHERE ses_id = '$ses_id'";
      $session_res = mssql_query($session_sql);
      $array = mssql_fetch_array($session_res);
      $ses_data = $array['0'];
      if ($session_res = "") {
            return '';
      }
      else return $ses_data;
}

/* Write new data to database */
function write($ses_id, $ses_data) {
      global $sql_table;
      $find_id_query = "SELECT ses_id from " . $sql_table . " WHERE ses_id = '$ses_id'";
      $result = mssql_query($find_id_query);
      $rows = mssql_num_rows($result);
      if ($rows >= 1) {
            $session_sql = "UPDATE " . $sql_table
            . " SET ses_time='" . time()
            . "', ses_value='$ses_data' WHERE ses_id='$ses_id'";
            $session_res = mssql_query ($session_sql);
            if (!$session_res) {
                  return FALSE;
            }
            elseif ($session_res == "1") {
                  return TRUE;
            }
      }
      else {
            $session_sql = "INSERT INTO " . $sql_table
            . " (ses_id, ses_time, ses_start, ses_value)"
            . " VALUES ('$ses_id', '" . time()
            . "', '" . time() . "', '$ses_data')";
            $session_res = mssql_query ($session_sql);
            if ($session_res == "1") {
            return (true);
            }
            else {
            return (false);
            }
      };
}            
            

/* Destroy session record in database */
function destroy($ses_id) {
      global $sql_table;
      $session_sql = "DELETE FROM " . $sql_table
      . " WHERE ses_id = '$ses_id'";
      $session_res = mssql_query ($session_sql);
      if (!$session_res) {
            return FALSE;
      }         else {
            return TRUE;
      }
}

/* Garbage collection, deletes old sessions */
function garbage($life) {
      global $sql_table;
      global $lifetime;
      $timenow = time();
      $ses_life = $timenow - $lifetime;
    $session_sql = "DELETE FROM " . $sql_table
                     . " WHERE ses_time < $ses_life";
      $session_res = mssql_query ($session_sql);
      if (!$session_res) {
            return FALSE;
      }         else {
            return TRUE;
      }
}

session_set_save_handler("open","close","read","write","destroy","garbage");
session_start();

This is for mssql so change accordingly if you use MySQL.
I saved this file as session.inc then used require_once in the main section of the site.
Works like a charm!
Hope this helps.
D.