window-sides-slots

window-sides-slots is a variable defined in `window.el'.
Its value is

(nil nil nil nil)

• This variable may be risky if used as a file-local variable.

Documentation:
Maximum number of side window slots.
The value is a list of four elements specifying the number of
side window slots on (in this order) the left, top, right and
bottom side of each frame. If an element is a number, this means
to display at most that many side windows on the corresponding
side. If an element is nil, this means there's no bound on the
number of slots on that side.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.