Right now, we have drivers for all of the miles the trucks go.
Even if we posit that there may be more trucks on the road with self-driving long-haul routes, it's hard to imagine that it's enough more to make up for all the long-haul truckers who will no longer be needed. And at least for most purposes, it seems highly likely that the number of last-mile deliveries is primarily driven by demand, not the availability of long-haul truck routes.
Even if they could increase slightly, there are limits to how many trucks the highways can fit without congesting pretty badly. (Anecdotally, the NYS Thruway is already approaching that limit at some times and in some areas.)
So how would we end up with more drivers total than now?
Most of them get paid $0.X per mile, I imagine the interstate miles are the easy money and the last mile through Manhattan is the hell money.