Will open files limit in Centos affect HTTP connections? Does the limit apply to a single session or all sessions?
ulimit is a bash builtin, so it applies only to the shell, and particularly any processes created by the current shell. Hence if you change it for any reason you will have to do so in a persistent way.
You might want to checkout the /etc/security/limits.conf file for setting session limits on a per-user basis. (which appears to be the CentOS preferred method)
you can see open files using the lsof -P | grep $somepid command