buffers-menu-max-size

buffers-menu-max-size is a variable defined in `menu-bar.el'.
Its value is
10


Documentation:
Maximum number of entries which may appear on the Buffers menu.
If this is 10, then only the ten most-recently-selected buffers are shown.
If this is nil, then all buffers are shown.
A large number or nil slows down menu responsiveness.

You can customize this variable.