There will be room for around 100,000 fans depending on the stage set-up at U.S. Bank Stadium, per Wendy Blackshaw, president/CEO of Minnesota Sports and Events. There will also be watch parties around the region that will count toward total attendance as well.
Many have asked how local organizers will squeeze hundreds of thousands of NFL draft fans around U.S. Bank Stadium. I don't entirely know, but remember there is a 4.2-acre park adjacent to it. (Via MSE)