Server reached MaxRequestWorkers setting, consider raising the MaxRequestWorkers setting

There are three modules related to this settings. 
prefork, worker and event

First find which module is activated using

a2query -M

Now change its conf file in mods-available. 
In following case, prefork is enabled, so make changes in mpm_prefork.conf

Open file mpm_prefork.conf and increase the MaxRequestWorkers settings according to your RAM.

MaxRequestWorkers
A simple calculation for MaxRequestWorkers would be: 
(Total Memory – Critical Services Memory) / Size Per Apache process

In this case the server has 3400Mb of memory allocated, 
250Mb used by mySQL and each Apache thread averages about 30Mb. 
MaxRequestWorkers = (3400– 250) / 30
therefore MaxRequestWorkers = 105

Restart Apache. Check here too.