Script to determine Apache ServerLimit and MaxClients.

check-apache-instances.sh

#!/bin/sh
ps -ylC httpd --sort:rss | awk '{ s += $8; } END \
  { print "Average Size:", s/NR/1024, "MB,", NR, \
  "servers, Total usage:", (s/NR/1024)*NR, "MB, Max Servers:", \
  6500/(s/NR/1024) }'

httpd.conf

<IfModule prefork.c>
    StartServers            10
    MinSpareServers         10
    MaxSpareServers         25
    ServerLimit             128
    MaxClients              128
    MaxRequestsPerChild     0
</IfModule>

shell-output.txt

$ free -m
             total       used       free     shared    buffers     cached
Mem:          7454       1405       6049          0        142        401
-/+ buffers/cache:        860       6594
Swap:         1023          0       1023

$ ./check-apache-instances.sh
Average Size: 26.043 MB, 37 servers, Total usage: 963.59 MB, Max Servers: 249.588