True! Presumably they'd refuel the vehicles in these places as well. The number of depots would depend on how much time you want to waste having vehicles driving out-of-direction and (1) the cost of additional real estate plus (2) the cost of employees at each location.
They're electric cars, so I assume they don't need to be juiced up in between rides.
The service runs 24/7, the vehicles certainly have to be out of service to charge at some point.