From the Nation, Sept. 29, 2011:

How many people have responded to the Adbusters call? How large is the group? And how large has it ever been?

The original Adbusters call envisioned 20,000 people flooding the Financial District on September 17. A tenth of that probably ended up being there that day. Despite a massive Anonymous-powered online social media blitz, lots of people simply didn’t know about it, and traditional progressive organizations like labor unions and peace groups were uncomfortable signing on to so amorphous an action. Over the course of a difficult first week, with arrests happening just about every day, new faces kept coming, as others filtered out to take a break. The media coverage after last weekend’s mass arrests and alleged police brutality has brought many more. Now, during the day and into the night, one finds 500 or more people in the plaza, and maybe half that sleeping over.* At any given time, several thousand people around the world are watching the occupation’s 24/7 livestream online.
http://www.thenation.com/article/163719/occupy-wall-street-faq

*This was in September of last year. There are probably many fewer now in the grandly titled “OWS General Assembly.”