@starbreaker Because 'ulimit' is a shell built in command, it is typically documented there. (special commands that modify the shell's own environment, such as 'set', 'cd' and 'ulimit' are almost always shell builtins by necessity)

If your shell is ksh (some googling suggests it may be the default interactive shell on openbsd), try here, which does document "ulimit -n" tersely: http://man.openbsd.org/ksh.1