Set the -iqgovern switch to specify the number of concurrent queries on a particular server. This is not the same as the number of connections, which is controlled by your license.
By specifying the -iqgovern switch, you can help IQ optimize paging of buffer data out to disk, and avoid over committing memory. The default value of -iqgovern is (2 x the number of CPUs) + 10. You may need to experiment to find an ideal value. For sites with large numbers of active connections, try setting -iqgovern slightly lower.