check_file_descriptors [-v]
The number of open files is likely to be larger than you expect, because each process probably has a number of dynamic libraries open, in addition to files which it opened explicitly.
It is quite painful to run out of file descriptors: you cannot start any new processes, the behavior of standard utilities like terminal and tcsh seem erratic, and Antelope programs have varied behavior. If you happen to be logged out, you may not be able to get logged in.
The best thing is to avoid this situation altogether. check_file_descriptors is run periodically by rtexec to watch for the problem, but you can also run it directly.
% check_file_descriptors -v 3968 fds open of 10000 maximum leaving 6032 available 927 cdorb2db 191 Mail 178 WebProces 126 tcsh 102 iTunes 97 Dropbox 95 Instrumen 94 perl 91 Dashboard 91 launchd . . .
lsof(8)