Why worry about how many customers? Surely the only important criterion is turnover, whether it's from one customer or 100.
Decide how much you want each van to turnover and tell them they have to hit that target each day - always assuming you have enough work to achieve it.
Our lads are all Franchisees, so we have an agreed target with each one. We allow 15% for 'not todays' etc so expect them to achieve 85% of the target they asked us for on average over a set period.
Nice and simple, and works for us