You can count that as 160 BPM or 320 BPM. At 160 BPM, the banjo is playing primarily 16th notes, at 320 the banjo is playing primarily 8th notes. Of course in either case, the banjo is playing the exact same speed… it’s just how someone wants to count it.
No matter how you count it, there are a barrage of notes flying out of the banjo, and that’s part of why it is so compelling. Not everyone can play at any given speed, but you might be surprised how quickly people can pick up speed as they learn banjo.