Random googling gives the size of a mediaeval town (Constance, in 1300 A.D.) of like 8,000, and 140 members of the city council. So I dunno whether they strictly need it -- probably not. But as you would expect, having bloated governing structures is a thing since governing structures were a thing. So, eh.

@Tasoli: Point taken, but the scene in question describes a court trial. So even if the jury consisted out of a subcomittee, why would they need the other members there? Iunno. It's not really important, though. Perhaps it is more spectators, but VIP seats, yeah. Regardless of whether that is the rest of a larger Wizengamot, or otherwise relevant people.