This is probably the number one feature request during the D6 cycle of Display suite: limit the number of items on a multiple value field, usually on images. This involves custom coding in the form of creating new formatters over and over again in code or using the Custom formatters module. The upcoming release of Display Suite for Drupal 7 now puts an end to this annoying limitation. All Field API fields with a cardinality set to unlimited or more than one get an extra textfield on the manage display screens to limit the output per field. I've recorded a short screencast to demonstrate that exciting new suble feature. A new release is scheduled this wednesday, so test and report any errors when found!