mouse-buffer-menu-maxlen

mouse-buffer-menu-maxlen is a variable defined in `mouse.el'.
Its value is

20

Documentation:
Number of buffers in one pane (submenu) of the buffer menu.
If we have lots of buffers, divide them into groups of
`mouse-buffer-menu-maxlen' and make a pane (or submenu) for each one.

You can customize this variable.