Is there a limit in the clients connections

I have been making some performance tests with OpenJMS 0.7.7 and found
that when the number of client connections increases (about 20) a
saturation seems to appear and messages are not received any more.
I was using pub/subscribe scheme.
The server is in a computer, and publisher and consumers in two other
different computers, all are running Win2k.
Is there any maximum number of simultaneous connections? If so, could it
be changed (increased).
Can I change any configuration parameter to increase the performance of
the server?

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