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.