The Apache-webserver starts in out configuration a CGI-script for some requests. That is a shell-script, that basically starts a mapserver with some parameters. As the mapserver-process uses many resources, we want to limit the number of processes started at the same time.
SnapOverflow
apart of doing some logic into your scripts you use some mechanism that is built into apache see http://www.cyberciti.biz/tips/linux-apache-setting-perl-cgi-script-limits.html