When in doubt, head to the man pages.
$ man -k nfs is a good place to start. It lists (amongst other things)
nfsd(8). Reading that man pages shows the
-n option. As system services are started by
rc.conf(5), and
/etc/defaults/rc.conf lists pretty much everything that one can use, a quick grep for nfsd gives
Code:
nfs_server_flags="-u -t -n 4" # Flags to nfsd (if enabled).
Just add that line to /etc/rc.conf and set as needed. Finally, restart nfsd using the script in /etc/rc.d/