There was a census worker. He trekked several miles down a long path through the woods. He came across an old man, sitting in a rocker. The old man asked, "Who are you? Why are you here?"
The man replied, "I'm from the census bureau, and I'm trying to find out how many people live in the U.S."
"Well, I don't why you would come back here and ask me, I don't know"
I think it boils down to a definition of "trader." There are millions of retail brokerage accounts (I've seen 13-18million mentioned), but how many make even 10 trades per month? And, keep in mind, the average share size at some place like Scottrade is between 100-200 shares, not really trading material.