Your choice of leader(s) completely depends on what you envision for the chopped material, as well as what happens while you're growing the material, out, as sometimes your vision will change as the material grows and develops over time. Leaders are used to grow sections of trunk, and lots of chopped material have years and years of growing on before the single main trunk has achieved the approximate size and shape to move on to the refinement phase. However, if the material might allow for a multi-trunked design, or a tree with multiple sub trunks, you'd allow multiple leaders to grow. Your example above is basically an informal upright styled with a single, short but thick trunk and multiple sub-trunks extending out to create the canopy. I suppose, if this tree was chopped, the artist could have decided on keeping a single leader/trunk and growing on from there. There really isn't a right or wrong answer. I will say that I like this tree a lot and applaud the decisions that were made to create it. Give this one another 10-15 years to thicken those sub trunks a bit and it'll look much older than it does right now.