VPN Connection

Basically i am a stock broker. I have created a vpn server at my place using windows 2003 server. i have succesfully created the username and passwords for others to connect to my system through internet.

Practically i have created 40 such usernames and want to connect all those to my server at the same time.

now the problem that i am facing is any of the 5 users can connect sucessfully but others cant once any one has disconnected the other can connect. but at a time all the users cannot connect.

waiting for a solution from you with great hopes. please help me to solve this problem at your earliest.