As lighttpd is a single-threaded server, its main resource limit is the number of file descriptors, which is set to 1024 by default (on most systems).
If you are running a high-traffic site you might want to increase this limit by setting server.max-fds.
Changing this setting requires root permissions on startup.
By default lighttpd would not change the operation system default. But setting it to 2048 is a better default for busy servers.
Default: ulimit -n