I am pretty sure the game only goes to 18, because when you get to a 18, the chart just becomes a retarded mash-up of steps, and thats as high as it has gotten.
This is a 18...
YouTube - OMEGA Plays DEAD END (Groove Radar Special) on SuperNOVA 2
Also 888 is not a 20, its probably a 15 or 16, its not that hard, its just requires you to hit many arrows, and a pretty fast speed in a short period of time, and the chart is not very hard. I have full combo'd it before on in the groove, and got a decent score on it.