I don't think the number would rise, at least not for a long time. Trucks would be utilized a lot more hours per day, which is what matters. There are currently pretty strict rules for how many hours a driver can work, but if you can sleep while the truck drives itself all of a sudden you have an asset that can work nearly 24/7.
Obviously, but by working more hours a day supply increases even though there aren't more trucks. The same thing will happen with large pools of shared autonomous cars--each car doing more work per day even though there are less cars.
More hours per day equates to more trucks total. If the transit between San Fransisco and Nashville suddenly takes half as long, we can reasonably assume that the number of trucks between San Francisco and Nashville will rise (maybe not double, but certainly a lot, and with reduced costs, who knows, maybe double or more).
But hardly enough to keep all current drivers employed.