"unbound[12941:0] warning: too many file descriptors" WHEN num-threads: 2

When setting 

num-threads: 2

in /etc/unbound/unbound.conf

I get this error and unbound isn't willing to start:

[1380316227] unbound[12941:0] warning: too many file descriptors
requested. The builtinmini-event cannot handle more than 1024. Config
for less fds or compile with libevent [1380316227] unbound[12941:0]
warning: continuing with less udp ports: 477 [1380316227]
unbound[12941:0] error: bind: address already in use [1380316227]
unbound[12941:0] fatal error: could not open
ports /etc/rc.d/local_unbound: WARNING: failed to start local_unbound

Why can't I set the number of threads?