Unable to connect to oracle if more than 10 users are logged on

Hi

I am faced with the problem of the error
TNS-12519 TNS:no appropriate service handler found, which keeps appearing every now and again when more than 10 users use the Project we are currently using in our company.
For your information, our project is built using Visual basic and Oracle 9i.
Is there any way by which we could increase the no of sessions/instances so that there is no bottleneck when a number of users access oracle at the same time. Should this be done in the Oracle registry or should the instance of oracle be altered. We are in urgent requirement of the same, as the network totally hangs when more than 10 users use our project. Earlier this problem wasnt there. But after the installation of the modified version of the project this seems to be very frequent, and we got to restart the server every now and again.
Please suggest the different ways to curb this problem and to get a way out.
PLEASE SUGGEST.
regs
Abigail