I think you are underestimating the annual mileage - an active driver would drive (at least) 250 days a year (5 days a week X 50 weeks a year). And they probably drive (at least) 500 miles a day, so that is 125,000 miles per year.
Apparently, they have gotten ~7,000 deposits:
http://insideevs.com/nikola-motor-lo...ic-semi-truck/