Set number of result to display with scrollbar in Ajax AutoCompleteExtender using CSS

public static List<string> SearchClients(string prefixText, int count)
        {
            List<string> customers = new List<string>();
            try
            {
                string strCon = System.Configuration.ConfigurationManager.AppSettings.Get("PLCConnectionString");
                string query = "select plc_clientname from plc_client where plc_active = '1' and plc_clientname like '" + prefixText + "%' order by plc_clientname";
                DataSet ds = SqlHelper.ExecuteDataset(strCon, CommandType.Text, query);
                if (ds.Tables.Count > 0)
                {
                    if (ds.Tables[0].Rows.Count > 0)
                    {
                        foreach (DataRow dr in ds.Tables[0].Rows)
                        {
                            customers.Add(dr["plc_clientname"].ToString());
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                string test = ex.Message;
            }
            return customers;
        }

Can Anyone help me that how to set limited number of result should display in list and rest should go on scroll

i m in hurry so please rply as much as possible