Thanks for your replies!
I'm editting the Apache Conf in /etc/httpd/httpd.conf (I think that's close to the correct path).
I edited the section where the "max clients" section is...
Is there another httpd.conf that runs with ISPconfig that I can adjust?
This would be a great help since ISPconfig "webserver" is always crashing, almost hourly. My sites become inaccessible!