INT 2fH 4a11H BX=0009H: Get Count of DISK_UNIT Structures

                                                         Compatibility: 6.0+ 
 Expects: AX    4a11H (mux number)
          BX    0009H (DoubleSpace fn code)
          DL    any compressed drive number (0=A, 1=B, etc.)
 Returns: AX    DoubleSpace API Error Code (0000H=success)
          CL    count of DISK_UNIT structures allocated by DBLSPACE.BIN
    Info: This determines how many compressed volumes can be mounted

          This is related to the value of the MaxRemovableDrives setting in
          DBLSPACE.INI.  This fn returns that number plus the number of
          successful ActivateDrive lines in DBLSPACE.INI.

See Also: DoubleSpace Overview
          DoubleSpace API
          INT 2fH: Multiplex Interrupt
          DOS Functions