I answered this in a different thread a couple of days ago, methinks. I can't find it now so real quick...
StartServers = How many child processes Apache fires up when it starts.
In the "default" configuration, Apache starts up one "controller." Once that's fired up, it forks off a series of worker processes. The number of worker processes started is what you'll define in StartServers.
There are other directives in the same family:
MinSpareServers.. how many will wait for requests at a minimum.
MaxSpareServers... the upper threshold at which apache will kill off unwanted children.
MaxRequestsPerChild... how many requests each child can handle before it's killed are respawned. Due to some memory leaks in older C libraries, for the most part, unlimited is fine now.
MaxClients... total number of servers we can start (can be above mss...)