Here is the general guidance we give in the Ubuntu installation instructions (http://webwork.maa.org/wiki/Installation_Manual_for_2.4_on_Ubuntu_8.04#Configuring_Apache):
Next we will make a few changes to Apache's default configuration. First we backup the configuration file
# cd /etc/apache2/
# cp apache2.conf apache2.conf.bak1
# gedit apache2.conf
Search for the line
and replace it by
Next search for the lines
Which occur under <IfModule mpm_prefork_module> and replace them by
# For WeBWorK a rough rule of thumb is 20 MaxClients per 1 GB of memory
where you should set MaxClients depending on the amount of memory your server has.
Note that the default MaxRequestsPerChild 0 means children never die. We would certainly appreciate guidance on the "For WeBWorK a rough rule of thumb is 20 MaxClients per 1 GB of memory".