Assuming most of the people do not trade (doctors lawyers etc) and give their money to brokers I would say generously 1 in 10 make decisions for themselves.
Which means when you are trading there are only 10% or 1.6 million people actively making decisions in the market. The liquidity you see is controlled by a very few select people. I would say 10% of that which would be about 160,000 large players.
Keep it all in perspective when you see volumes of 200 Million shares. Not really an accurate picture! ;)