In order to focus in on a particular topic, but with multiple copies to allow several people to choose the same image. Having more than 1 person choose the same image allows an exploration of how people can view the same thing, but have very different interpretations and reactions.
In order to work with a typical intact team size (12 people) and provide a good range of images to select. If people are ‘taking’ an image from a deck you want each person to feel they have a wide choice and someone didn’t ‘take’ their card first.
Large groups may require large amounts of images, but consider whether too many images may extend the duration of the activity, or whether the groups can be sub-divided with the same or different images. Consider placing all images for viewing on floor and have people walk amongst the images, providing significant time for inspection & reflection.