Today’s Chart comes from an article in Barron’s with information from Wilshire Indexes. The “Wiltshire 5000” is an index similar to the S&P 500, with the difference being that instead of the largest 500 stocks in the S&P, the Wiltshire 5000 represents all of the investable stocks in the United States. Some argue the Wilshire 5000 is a better representation of the health of the stock market, but that is a longer discussion for another day. 
As noted in the chart, that total number of investable stocks ebbs and flows over time with the high being 7,500 stocks in the late 1990s to the all-time low of only 3,700 today.
Side fact: There are an additional 6,400 stocks outside of the US, creating a grand total 10,100 stocks in the entire world.