Explanation of the FCBS command. It is CONFIG.SYS.

This will specify the number of file control blocks (FCBs)
that MS-DOS can have open at the same time. It has to be used
in the CONFIG.SYS file of your computer.

Example:
       To let MS-DOS work with 10 FCBs at a time you would have 
       to include the following line in your CONFIG.SYS file:
          fcbs=10