file-max && file-nr

/proc/sys/fs/file-max
          This  file defines a system-wide limit on the number of open files for all processes.  This limit is not applied
          to root.  (See also setrlimit(2), which can be used by a process to set the per-process limit, RLIMIT_NOFILE, on
          the  number  of  files  it  may open.)  If you get lots of error messages about running out of file handles, try
          increasing this value:

          echo 100000 > /proc/sys/fs/file-max

          The kernel constant NR_OPEN imposes an upper limit on the value that may be placed in file-max.

          If you increase /proc/sys/fs/file-max, be sure to increase /proc/sys/fs/inode-max to 3-4 times the new value  of
          /proc/sys/fs/file-max, or you will run out of inodes.

/proc/sys/fs/file-nr
          Historically,  the three values in file-nr denoted the number of allocated file handles, the number of allocated
          but unused file handles, and the maximum number of file handles. Linux 2.6 always reports 0  as  the  number  of
          free  file  handles  --  this  is  not an error, it just means that the number of allocated file handles exactly
          matches the number of used file handles.