It was bigger than an arena tour. In concert configuration, most basketball arenas do about 5k less than their basketball set-up, and all available seats aren't sold, sections will be blocked off. A lot of people doing arenas are doing 7-15k in a venue.
For instance, Drake's tour stop in Austin was in a basketball venue with a 16k basketball capacity, but he did 11k in it. A lot of 'sold out' arena show tours are in that range. Maybe more, maybe less.
J.Cole is going to be the first rapper from the 00's headlining a stadium tour. His fanbase supports him, heavy, and with no hype. People will forever continue to deny what Hov having him as his opener for his big stadium/festival tour years ago did for his career. He got him, a recording artist, in in front of hundreds and hundreds of thousands of people who pay money for music before he had an album out.