This is a good/important question.  The short answer is, all Rep-holders will not have to vote on every event, as this would of course not scale!  Instead, the number of reporters required to vote on an event scales with the volume traded in the event's market.  More volume traded = more reports needed.  Specific reporters are assigned to events at random, weighted by their Rep holdings.  For details, please see Joey's in-depth blog post on scaling Augur.