iswitchb-max-to-show

iswitchb-max-to-show is a variable defined in `iswitchb.el'.
Its value is

nil

Documentation:
If non-nil, limit the number of names shown in the minibuffer.
If this value is N, and N is greater than the number of matching
buffers, the first N/2 and the last N/2 matching buffers are
shown. This can greatly speed up iswitchb if you have a
multitude of buffers open.

You can customize this variable.