No, not by very much. Computerised dispatching systems tell trucks where to go, even with human operators, and a human is probably a little better at driving than an automated truck. The gain is that the trucks no longer park for any reason other than maintenance, while humans have to stop to embark/disembark. And will frequently have, eg, a fatigue break during night shift.
If anything, supply/demand curve logic suggests there would be more trucks overall.
As you say, existing trucks are parked a bunch of hours a day. I think you and the parent have a semantic misunderstanding. You (i think) are saying there will be more trucks on the road, more routes being run at the same time. Th parent is saying fewer trucks will be built, because we can get 100% utilization as opposed to the 33% we have now. [1]
[1] disclaimer: i don't think robot trucks will have 100% utilization, and current utilization is probably higher than 1/3, i just guessed 8 hours of use a day. I think the point stands, more trucks on the road at the same time, but fewer trucks overall.
I understand what the parent is saying, I just disagree :).
My day job has large elements of mining haul truck performance. Currently most mines would probably be in the 40-70% utilisation band (for practical purposes), automating the trucks would probably add ... 5% utilisation? Maybe 10%? Both guesses, but they aren't likely to break 75% (I doubt they would break 65%, but I'm wiling to be surprised by evidence).
Practically speaking, the trucks would be more reliable due to automating, and in economic terms, truck hours/dollar would increase (*edit, said drop initially). Supply/demand theory tells us that mining companies could reasonably respond by buying more trucks.
