Patch stated these high capacity event regions had seen some testing, notably at shopping events. He brought up that some had "cam regions" in which people had to look at the shopping stalls from outside, and these kinds of sims would make that a thing of the past. Testing had shown they could run a hundred avatars "safely." Patch was saying the goal was to have these sims hold up to 150 avatars, "and still be able to walk around and move and see everything." Patch felt it was "Mindblowing" to be in a sim with not 30-40 avatars around, but a hundred to a hundred fifty. He felt the technology could eventually be used to have "true large-scale auditorium style events beyond the capabilities of what we have today."
And the capacity of the Second Life grid? Patch stated the days of Second Life crashing when 100,000 were on are "behind us" thanks in part to the Amazon Cloud Servers, "We can go as big as we want. ... that'll be amazing once we get back to those numbers again."
Addition: When Patch was asked if high capacity sims could be part of this year's Relay Weekend, Patch answered "Maybe."