The flocks they use for a herding competition in Germany are usually reasonably large (around 200 sheep). They have very different breeds over there to the wild, flighty sheep you see here in Scotland and the North of England. The sheep are also well trained to follow the shepherd (person) and the GSD's job is to either keep all the sheep in a particular grazing area (akin to a "living fence") and when they move from area to area, the GSD keeps them away from traffic (he moves between the sheep and the moving traffic to keep them out of harm's way).
I think many people in the UK wrongly believe that a GSD herding is the same as a BC herding, and are then surprised to see the difference between the GSD and the BC - however, these breeds have completely different jobs. The GSDs do herd "but not as we know it"