max_clients []

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Quick note: apache 2.4 - Max Clients is now named MaxRequestWorkers
You can find the apache values in:
/etc/httpd/conf/httpd.conf
OR
/etc/httpd/conf.modules.d/00-mpm.conf
If no value has been set then chances are the default of 256 is set.

CentOS

Best command:

pstree | grep httpd

Alternative:

ps afx | grep httpd | wc

Output shows how many connections apache currently has

What is the number of max connection the server has been configured for?

grep -i maxclients /etc/httpd/conf/httpd.conf

When did apache hit max clients?

Centos

grep -i maxclients /var/log/httpd/error_log

Has it happened before?

zgrep -i maxclients /var/log/httpd/error_log*

What are the currently configured max connections?

grep -i maxc /etc/httpd/conf/httpd.conf | head -3

Ubuntu could have max clients configured in a number of different places, first place to look is:

grep -i maxc /etc/apache2/apache2.conf | head -4

or

grep -i maxc /etc/apache2/mods-enabled/mpm_prefork.conf

Current connections:

pstree | grep httpd

Ubuntu

grep -i maxclients /var/log/apache2/error_log

Has it happened before?

zgrep -i maxclients /var/log/apache2/error_log*

Current connections:

pstree | grep apache

CentOS

ps -H h -ylC httpd | perl -lane '$s+=$F[7];$i++;END{printf"Avg=%.1fMB\n",$s/$i/1024}'

Ubuntu

ps -H h -ylC apache2 | perl -lane '$s+=$F[7];$i++;END{printf"Avg=%.1fMB\n",$s/$i/1024}'