In Atlanta, critical mass got up to 300+ riders before the city/police finally got tired of the illegal blocking of intersections. (They call it "corking") Atlanta police had 10-20 motorcycle cops follow the ride and pull over anyone who broke the law (running red lights, etc.) for a couple of weeks and the popularity quickly dropped off when people realized they had to still follow traffic laws. It still goes on every month, but it's more like 50 riders now.
There are also of course plenty of legal rides/events that get permits, police escorts(sometimes free), etc. We have something called "Atlanta Streets Alive" that shuts down several miles of large streets every few months to only pedestrian and bike traffic, allowing people to walk and play in the streets with no cars. Permits are required, off-duty police have to be hired to direct traffic at intersections etc.
There are also of course plenty of legal rides/events that get permits, police escorts(sometimes free), etc. We have something called "Atlanta Streets Alive" that shuts down several miles of large streets every few months to only pedestrian and bike traffic, allowing people to walk and play in the streets with no cars. Permits are required, off-duty police have to be hired to direct traffic at intersections etc.