If you're using Stateless Session Beans as the backing logic for your WebServices, have you tried configuring the Bean Pool for your session bean to limit the number of bean instances created by the container?
If you don't allow more than 30 beans, you shouldn't be getting more than 30 simultaneous connections.
Mind, any other connections may well encounter a not very graceful failure, so it may be important to ensure that your clients will retry as necessary should they encounter a problem.
I have not tested this idea, but it should be simple enough to try out and see if it gives you what you are looking for.
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