Yeah, this was my thought as well. Not sure how accurate this article[0] is, but it says the trailer to power-unit(tractor) ratio is around 3-5:1.
One interesting thought though: if these trailers could implement thrust vectoring and other safety measures(like adjacent vehicle detection), it may be realistic to run longer trains than can be done with dumb trailers that tend to be unhandleable beyond 2-long. This may effect a reduction in the trailer pool ratio, increasing efficiency even more.
I grew up in the Columbia River gorge and I remember driving alongside triple-trailer pulls and it was terrifying to attempt passing them when the wind was gusty. I'm confident everyone that frequented that corridor could regale you with tales of being almost run off the road by one of these behemoths at least once.