On versions of FreeBSD before 3.0, the FD_SETSIZE define defaults to 256. This means that you will have trouble usefully using more than 256 file descriptors in Apache. This can be increased, but doing so can be tricky.
If you are using a version prior to 2.2, you need to recompile your kernel with a larger FD_SETSIZE. This can be done by adding a line such as:
options FD_SETSIZE nnn
to your kernel config file. Starting at version 2.2, this is no longer necessary.
If you are using a version of 2.1-stable from after 1997/03/10 or 2.2 or 3.0-current from before 1997/06/28, there is a limit in the resolver library that prevents it from using more file descriptors than what FD_SETSIZE is set to when libc is compiled. To increase this, you have to recompile libc with a higher FD_SETSIZE.
In FreeBSD 3.0, the default FD_SETSIZE has been increased to 1024 and the above limitation in the resolver library has been removed.
After you deal with the appropriate changes above, you can increase the setting of FD_SETSIZE at Apache compilation time by adding "-DFD_SETSIZE=nnn" to the EXTRA_CFLAGS line in your Configuration file.