If you have many matching files, they may not all fit onto one line of
the minibuffer. Normally, the minibuffer window will grow to show you
more of the matching files (depending on the value of the variables
resize-mini-windows and max-mini-window-height). If you
want Ido to behave differently from the default minibuffer resizing
behavior, set the variable ido-max-window-height.
Also, to improve the responsiveness of Ido, the maximum number of
matching items is limited to 12, but you can increase or remove this
limit via the ido-max-prospects user option.
To see a full list of all matching buffers in a separate buffer, hit ? or press TAB when there are no further completions to the substring. Repeated TAB presses will scroll you through this separate buffer.