The options that you might like to change are the blocksize and the number of blocks. The blocksize is the atomar unit of data MULTIBUF deals with. Some tape drivers (e.g. ZFTAPE ) require this to have a certain size, 10240 by default. The number of blocks determines how big the memory buffer is that each instance of MULTIBUF allocates. For more information, see also the sections 8.2.2 and 8.2.2 about double buffering.
Usually, you should not change any of these options. If you encounter problems with the MULTIBUF program, you can disable its use here.