Изменить количество дисплеев emacs ido

C-h v ido-max-prospects:

ido-max-prospects is a variable defined in `ido.el'.
Its value is 12

Documentation:
Upper limit of the prospect list if non-zero.
Zero means no limit for the prospect list.
For a long list of prospects, building the full list for the
minibuffer can take a non-negligible amount of time; setting this
variable reduces that time.

You can customize this variable.

(setq ido-max-prospects 5) ;; show 5 or fewer candidates in ido

Кстати, я нашел эту переменную, просмотрев все переменные в M-x customize-group ido.