Empty maybe during the prelims. The shows they've had in Europe were filled along with the Japan show with RIZIN. Mostly all the shows held in the USA are packed but not sold out. Then again even though the UFC has consistently claimed during PPV's the arena is sold out they are lying.
Just check Ticketmaster when the UFC PPV portion of the card starts you'll see plenty seats still available for the event this can easily be seen during the prelims portion of the card. During the PPV portion of the UFC event they move people from nose bleed section down so it has the illusion its packed then have the gall to claim the show is sold out but again Ticketmaster doesn't lie.