msb-max-file-menu-items

msb-max-file-menu-items is a variable defined in `msb.el'.
Its value is

10

Documentation:
The maximum number of items from different directories.

When the menu is of type `file by directory', this is the maximum
number of buffers that are clumped together from different
directories.

Set this to 1 if you want one menu per directory instead of clumping
them together.

If the value is not a number, then the value 10 is used.

You can customize this variable.