any idea to group shipment jobs in a fixed size by pickup & delivery location?

I am a newbie on Jsprit, my scenario is there having some shipment orders, and needs to be grouped by the order’s similar pickup and delivery location, and then assigned to a vehicle to deliver the group package or not. I tried to build dummy vehicles list based on the shipment orders size/group package size, and assume the shipment orders will be assigned to vehicles in a fixed size, but the problem is the Jsprit algorithm will always assign all the shipment orders to the same vehicle, and not in a balance way, any idea to solve the problem will be appreciated, thanks!