The ATA used to say the number was 3.5 million, without explanation. Now it says 3 million drivers as of 2010. I think they are just reporting the number of people who hold Commercial Driver's Licenses. The map in the article is animated in the NPR source, where they asked "What's with all the truck drivers?" Also: "Employment of heavy and tractor-trailer truck drivers is projected to grow 5 percent from 2014 to 2024, about as fast as the average of all occupations." Hmm.
the BLS says there's 1.8 million jobs in trucking
What started me looking into this was a conversation that a friend of mine had. His father works in a trucking company and they have stopped hiring new drivers because they anticipate self-driving trucks to be on the road a lot sooner than that. For the time being, there's going to be a human providing oversight in the seat, but considering that a truck could effectively run 24/7/365 without a normal driver's salary you can see how appealing that would be to a business to be a first-mover on.
I imagine there will be significant savings in insurance and accident costs as well. Next innovation: road trains.