"To configure more than 256 clients, one must edit the HARD_SERVER_LIMIT entry in httpd.h and recompile Apache.
"
see
http://www.apacheref.com/ref/http_core/MaxClients.html
MaxClients
Maximum Number of Clients
Syntax: MaxClients A
Example: MaxClients 128
Since: Apache 1.0
This directive limits the number of simultaneous HTTP requests that can be supported; no more than this number of child server processes will be created. To configure more than 256 clients, one must edit the HARD_SERVER_LIMIT entry in httpd.h and recompile Apache. Any connection attempts that exceed the MaxClients limit will normally be queued in kernel space, up to a number based on the ListenBacklog directive. Once a child process is freed at the end of a different request, the queued connection will be serviced.