Following today's sell-off in the equity market, there are now 226 stocks in the S&P 500 that are currently in oversold territory (more than one standard deviation below 50-day moving average). On a percentage basis, this works out to 45.2% of the stocks in the index. The last time there were more oversold stocks in the S&P 500 was back on October 4th.
Granted, the current sell-off hasn't been nearly as bad as the one we saw last September (yet), but because of the lack of volatility in the market over the last several months, the trading range for individual stocks became much narrower leading up to the current sell-off. Therefore, it didn't take as big of a sell-off to move the market and individual stocks into oversold levels.
To illustrate this, let's use the S&P 500 as an example. The chart below shows the price of the S&P 500 along with its trading range of one standard deviation above and below its 50-day moving average (gray region). In the lower chart we show the size of the trading range in percentage points from top to bottom. Back in early October, the market was significantly more volatile than it is today. As a result, the size of the S&P 500's trading range was above 9% and coming down from as high as 12.5%. Today, after months of a relatively placid market environment, the size of the S&P 500's trading range from top to bottom is 4.14%, or less than half of what it was then.
