I want to know where MaxClients constant is declarated or in which part of the source read this value of httpd.conf.
It isn't a "constant".
apache-1.3]$ grep MaxClients src/main/*.c
src/main/http_core.c: fprintf(stderr, "WARNING: MaxClients of %d exceeds compile time limit "
src/main/http_core.c: fprintf(stderr, " lowering MaxClients to %d. To increase, please "
src/main/http_core.c: fprintf(stderr, "WARNING: Require MaxClients > 0, setting to 1\n");
src/main/http_core.c: "Deprecated equivalent to MaxClients" },
src/main/http_core.c:{ "MaxClients", set_server_limit, NULL, RSRC_CONF, TAKE1,
src/main/http_main.c: * to deal with MaxClients changes across SIGUSR1 restarts. We use this
src/main/http_main.c: "server reached MaxClients setting, consider"
src/main/http_main.c: " raising the MaxClients setting");
It looks like the set_server_limit() function in src/main/http_core.c will process the MaxClients directive and set a global variable to represent the value. Have a look at that code.
