If the clients are using 2000 or XP pro, there is no limit. You don't need to purchase any additional TS CALs for those clients.
You may want to try disabling the license logging service. This service tracks Windows CALs (as opposed to TS CALs), which may be preventing more than 5 people from connecting. Keep in mind that, to be legal, you would need to buy more Windows licenses.
Disabling the license logging service will give you unlimited connections, and is perfectly legal to do. Microsoft disables this service by default in 2003 and it will not be in Longhorn. But again, you need to have enough hard copy Windows licenses to cover access to your server. But you don't need to buy any TS CALs for 2k and XP Pro clients.