With the LEGO SERIOUS PLAY method, we use the power of the LEGO bricks to achieve rich and meaningful communication between participants.
The LEGO group has produced an amazing diversity of minifigures. It seems obvious to use these to let our participants express fine and detailed metaphors. And sometimes it works very well for that purpose.
In many cases though, participants will use a fair amount of their time just adding more and more minifigures to their model, in order to represent a large group (for instance), or the diversity within a group, whereas a few bricks of varied colour would perfectly do.
Maybe surprisingly, sometimes it works even better without minifigures. Reason for that is that it drives the builder to focus on the qualities or the character of whom they want to tell the story. Using the flow theory as a reference, you could choose to offer less minifigures, or even none. In doing that, you use scarcity to make the challenge more difficult. Do it if you feel your participants can take it, learn more and express more as a result.