Directive Description Default (worker.MPM)
===================== =============================================================================== ====================
ListenBackLog Maximum length of the queue of pending connections 511
.
MaxClients Global maximum number of connections that will be processed simultaneously 400
MaxClients SHOULD NOT be lower than (ServerLimit) x (ThreadsPerChild).
Normally they should be equal.
.
ServerLimit Upper limit on configurable number of processes (children) 16
There is a hard limit of ServerLimit 20000 compiled into the server
.
ThreadLimit Sets the upper limit on the configurable number of 'ThreadsPerChild' 64
.
ThreadsPerChild Number of threads created by each child process 25
The child creates these threads at startup and never creates more.
This values can go up to value of ThreadLimit.
Normally this value should equal to ThreadLimit
to prevent reserving unused shared memory space.
.
StartServers Number of child server processes created at startup 3
.
StartThreads Total number of threads created at startup to serve requests.
Number of threads created on startup. As the number of threads is
dynamically controlled depending on the load,
there is usually little reason to adjust this parameter.
.
MinSpareThreads Minimum number of idle threads available to handle request spikes 75
MaxSpareThreads Maximum number of idle threads 250
.
MaxRequestsPerChild Sets the limit on the number of requests an individual child server handles 10000
in its life. After MaxRequestsPerChild requests, the child process will die.
If MaxRequestsPerChild is 0, then the process will never die.

So when Apache starts, it starts a number of Children processes that is: StartThreads divide by ThreadsPerChild.
It will then serve clients by creating dynamically children each providing (ThreadsPerChild) number of threads
up to a maximum of children (ServerLimit) or lower limited by MaxClients. When a child has processed
a (MaxRequestsPerChild) number of requests it dies and another child is created if needed.