Traditionally, the answer is 500, as the index was constructed using the 500 largest companies. However, Google's upcoming stock dividend will change all this. As of April 3, 2014, Google is undergoing a 2:1 split via a stock dividend, as owners of record will receive an additional share -- but of a different nonvoting class stock. This means that there will be two Google share classes being traded. To keep the value in place, the S&P will retain both share classes, meaning there will now be 501 stocks in the S&P500. See article and related video here, CNBC.
Dolvin, Steven D., "How Many Stocks Are in the S&P500?" (2014). All Chapters. 98.