https://www.sec.gov/news/studies/2010/marketevents-report.pdf I couldn't find notes on whether this count was exhaustive or a small sample, but since this was an investigation into the roots of the issue, I'm assuming they took a look at everybody involved that day. On page 29, it reveals there were about 10k traders before the 6th and about 15k on the 6th, categorizing them as HFTs, market makers, fundamental buyers/sellers, and Opportunistic and Noise traders. I guess I imagined the number of ES traders being closer to 100k, but 10k points to quite a small population...perhaps since there's such a high turnover in this field...
I wonder what percentage were institutional traders and how many were individuals. Page 2 says that a mutual fund company sold 75k E Mini contracts in 20 minutes......holy cow.
So.... essentially the article says that while Greece announced they may go bankrupt, nobody was looking to buy the S&P 500 ? Wow.... and 1 week later the market traded at the low of the "flash crash"....
It took the CFTC/SEC only 5 months to compile and write the report - it's been out since Sep 2010. Certainly an interesting read, the 30s and 40s are a great section discussing the drop in liquidity in the market leading up to the crash.