I was there today. I didn't know the rally was today though. I got a really good parking space though. Just be nice to the police and laugh at their jokes about the crowd, and they'll move a barrier one parking spot back and let you park there.
There were thousands there, they just used the trip as a mini vacation and went to all the Smithsonian Museums. Made for longer lines for me and having to listen to them talk about why they were in DC. There was only a small crowd for Al Sharpton. There were lots of "anarchist and socialists" with lots of signs and chants but no substance when questioned.