ido-max-work-file-list

ido-max-work-file-list is a variable defined in `ido.el'.
Its value is

10

Documentation:
Maximum number of names of recently opened files to record.
This is the list of the file names (sans directory) which have most recently
been opened. See `ido-work-file-list' and `ido-save-directory-list-file'.

You can customize this variable.